doc: Documentation in Markdown for Depends Dir

Documentation more readable when viewed on Github.

Some extra changes by @laanwj:

- Make README.usage the default README. This is more convenient from a
  user perspective. Link to other documentation in this default README

- Add list of popular targets for cross compilation, change default to
  Win64 instead of Win32

4 files changed, 137 insertions, 100 deletions

diff --git a/depends/README.md b/depends/README.md
new file mode 100644
index 0000000000..2dc0b9e47e
--- /dev/null
+++ b/depends/README.md
@@ -0,0 +1,56 @@
+### Usage
+
+To build dependencies for the current arch+OS:
+
+    make
+
+To build for another arch/OS:
+
+    make HOST=host-platform-triplet
+
+For example:
+
+    make HOST=x86_64-w64-mingw32 -j4
+
+A prefix will be generated that's suitable for plugging into Bitcoin's
+configure. In the above example, a dir named i686-w64-mingw32 will be
+created. To use it for Bitcoin:
+
+    ./configure --prefix=`pwd`/depends/x86_64-w64-mingw32
+
+Common `host-platform-triplets` for cross compilation are:
+
+- `i686-w64-mingw32` for Win32
+- `x86_64-w64-mingw32` for Win64
+- `x86_64-apple-darwin11` for MacOSX
+- `arm-linux-gnueabihf` for Linux ARM
+
+No other options are needed, the paths are automatically configured.
+
+Dependency Options:
+The following can be set when running make: make FOO=bar
+
+    SOURCES_PATH: downloaded sources will be placed here
+    BASE_CACHE: built packages will be placed here
+    SDK_PATH: Path where sdk's can be found (used by OSX)
+    FALLBACK_DOWNLOAD_PATH: If a source file can't be fetched, try here before giving up
+    NO_QT: Don't download/build/cache qt and its dependencies
+    NO_WALLET: Don't download/build/cache libs needed to enable the wallet
+    NO_UPNP: Don't download/build/cache packages needed for enabling upnp
+    DEBUG: disable some optimizations and enable more runtime checking
+
+If some packages are not built, for example `make NO_WALLET=1`, the appropriate
+options will be passed to bitcoin's configure. In this case, `--disable-wallet`.
+
+Additional targets:
+
+    download: run 'make download' to fetch all sources without building them
+    download-osx: run 'make download-osx' to fetch all sources needed for osx builds
+    download-win: run 'make download-win' to fetch all sources needed for win builds
+    download-linux: run 'make download-linux' to fetch all sources needed for linux builds
+
+### Other documentation
+
+- [description.md](description.md): General description of the depends system
+- [packages.md](packages.md): Steps for adding packages
+
diff --git a/depends/README.usage b/depends/README.usage
deleted file mode 100644
index 24e1231d82..0000000000
--- a/depends/README.usage
+++ /dev/null
@@ -1,34 +0,0 @@
-To build dependencies for the current arch+OS:
-    make
-To build for another arch/OS:
-  make HOST=host-platform-triplet && make HOST=host-platform-triplet
-  (For example: make HOST=i686-w64-mingw32 -j4)
-
-A prefix will be generated that's suitable for plugging into Bitcoin's
-configure. In the above example, a dir named i686-w64-mingw32 will be
-created. To use it for Bitcoin:
-
-./configure --prefix=`pwd`/depends/i686-w64-mingw32
-
-No other options are needed, the paths are automatically configured.
-
-Dependency Options:
-The following can be set when running make: make FOO=bar
-
-SOURCES_PATH: downloaded sources will be placed here
-BASE_CACHE: built packages will be placed here
-SDK_PATH: Path where sdk's can be found (used by OSX)
-FALLBACK_DOWNLOAD_PATH: If a source file can't be fetched, try here before giving up
-NO_QT: Don't download/build/cache qt and its dependencies
-NO_WALLET: Don't download/build/cache libs needed to enable the wallet
-NO_UPNP: Don't download/build/cache packages needed for enabling upnp
-DEBUG: disable some optimizations and enable more runtime checking
-
-If some packages are not built, for example 'make NO_WALLET=1', the appropriate
-options will be passed to bitcoin's configure. In this case, --disable-wallet.
-
-Additional targets:
-download: run 'make download' to fetch all sources without building them
-download-osx: run 'make download-osx' to fetch all sources needed for osx builds
-download-win: run 'make download-win' to fetch all sources needed for win builds
-download-linux: run 'make download-linux' to fetch all sources needed for linux builds
diff --git a/depends/README b/depends/description.md
index 55e7222697..74f9ef3f20 100644
--- a/depends/README
+++ b/depends/description.md
@@ -1,9 +1,7 @@
-This is a system of building and caching dependencies necessary for building
-Bitcoin.
-
+This is a system of building and caching dependencies necessary for building Bitcoin. 
 There are several features that make it different from most similar systems:
 
-- It is designed to be builder and host agnostic
+### It is designed to be builder and host agnostic
 
 In theory, binaries for any target OS/architecture can be created, from a
 builder running any OS/architecture. In practice, build-side tools must be
@@ -11,18 +9,18 @@ specified when the defaults don't fit, and packages must be amended to work
 on new hosts. For now, a build architecture of x86_64 is assumed, either on
 Linux or OSX.
 
-- No reliance on timestamps
+### No reliance on timestamps
 
 File presence is used to determine what needs to be built. This makes the
 results distributable and easily digestable by automated builders.
 
-- Each build only has its specified dependencies available at build-time.
+### Each build only has its specified dependencies available at build-time.
 
 For each build, the sysroot is wiped and the (recursive) dependencies are
 installed. This makes each build deterministic, since there will never be any
 unknown files available to cause side-effects.
 
-- Each package is cached and only rebuilt as needed.
+### Each package is cached and only rebuilt as needed.
 
 Before building, a unique build-id is generated for each package. This id
 consists of a hash of all files used to build the package (Makefiles, packages,
@@ -32,7 +30,7 @@ any other package that depends on it. If any of the main makefiles (Makefile,
 funcs.mk, etc) are changed, all packages will be rebuilt. After building, the
 results are cached into a tarball that can be re-used and distributed.
 
-- Package build results are (relatively) deterministic.
+### Package build results are (relatively) deterministic.
 
 Each package is configured and patched so that it will yield the same
 build-results with each consequent build, within a reasonable set of
@@ -41,13 +39,13 @@ beyond the scope of this system. Additionally, the toolchain itself must be
 capable of deterministic results. When revisions are properly bumped, a cached
 build should represent an exact single payload.
 
-- Sources are fetched and verified automatically
+### Sources are fetched and verified automatically
 
 Each package must define its source location and checksum. The build will fail
 if the fetched source does not match. Sources may be pre-seeded and/or cached
 as desired.
 
-- Self-cleaning
+### Self-cleaning
 
 Build and staging dirs are wiped after use, and any previous version of a
 cached result is removed following a successful build. Automated builders
diff --git a/depends/README.packages b/depends/packages.md
index 6d87402ebc..7c80362509 100644
--- a/depends/README.packages
+++ b/depends/packages.md
@@ -4,121 +4,137 @@ variables, and defining build commands.
 The package "mylib" will be used here as an example
 
 General tips:
-mylib_foo is written as $(package)_foo in order to make recipes more similar.
+- mylib_foo is written as $(package)_foo in order to make recipes more similar.
 
-Identifiers:
+## Identifiers
 Each package is required to define at least these variables:
-  $(package)_version:
+
+    $(package)_version:
     Version of the upstream library or program. If there is no version, a
     placeholder such as 1.0 can be used.
-  $(package)_download_path:
+
+    $(package)_download_path:
     Location of the upstream source, without the file-name. Usually http or
     ftp.
-  $(package)_file_name:
+
+    $(package)_file_name:
     The upstream source filename available at the download path.
-  $(package)_sha256_hash:
+
+    $(package)_sha256_hash:
     The sha256 hash of the upstream file
 
 These variables are optional:
-  $(package)_build_subdir:
+
+    $(package)_build_subdir:
     cd to this dir before running configure/build/stage commands.
-  $(package)_download_file:
+    
+    $(package)_download_file:
     The file-name of the upstream source if it differs from how it should be
     stored locally. This can be used to avoid storing file-names with strange
     characters.
-  $(package)_dependencies:
+    
+    $(package)_dependencies:
     Names of any other packages that this one depends on.
-  $(package)_patches:
+    
+    $(package)_patches:
     Filenames of any patches needed to build the package
-  $(package)_extra_sources:
+
+    $(package)_extra_sources:
     Any extra files that will be fetched via $(package)_fetch_cmds. These are
     specified so that they can be fetched and verified via 'make download'.
 
-Build Variables:
+
+## Build Variables:
 After defining the main identifiers, build variables may be added or customized
 before running the build commands. They should be added to a function called
 $(package)_set_vars. For example:
 
-define $(package)_set_vars
-...
-endef
+    define $(package)_set_vars
+    ...
+    endef
 
 Most variables can be prefixed with the host, architecture, or both, to make
 the modifications specific to that case. For example:
 
-  Universal:         $(package)_cc=gcc
-  Linux only:        $(package)_linux_cc=gcc
-  x86_64 only:       $(package)_x86_64_cc = gcc
-  x86_64 linux only: $(package)_x86_64_linux_cc = gcc
+    Universal:     $(package)_cc=gcc
+    Linux only:    $(package)_linux_cc=gcc
+    x86_64 only:       $(package)_x86_64_cc = gcc
+    x86_64 linux only: $(package)_x86_64_linux_cc = gcc
 
 These variables may be set to override or append their default values.
-  $(package)_cc
-  $(package)_cxx
-  $(package)_objc
-  $(package)_objcxx
-  $(package)_ar
-  $(package)_ranlib
-  $(package)_libtool
-  $(package)_nm
-  $(package)_cflags
-  $(package)_cxxflags
-  $(package)_ldflags
-  $(package)_cppflags
-  $(package)_config_env
-  $(package)_build_env
-  $(package)_stage_env
-  $(package)_build_opts
-  $(package)_config_opts
+
+    $(package)_cc
+    $(package)_cxx
+    $(package)_objc
+    $(package)_objcxx
+    $(package)_ar
+    $(package)_ranlib
+    $(package)_libtool
+    $(package)_nm
+    $(package)_cflags
+    $(package)_cxxflags
+    $(package)_ldflags
+    $(package)_cppflags
+    $(package)_config_env
+    $(package)_build_env
+    $(package)_stage_env
+    $(package)_build_opts
+    $(package)_config_opts
 
 The *_env variables are used to add environment variables to the respective
 commands.
 
 Many variables respect a debug/release suffix as well, in order to use them for
 only the appropriate build config. For example:
-  $(package)_cflags_release = -O3
-  $(package)_cflags_i686_debug = -g
-  $(package)_config_opts_release = --disable-debug
+
+    $(package)_cflags_release = -O3
+    $(package)_cflags_i686_debug = -g
+    $(package)_config_opts_release = --disable-debug
 
 These will be used in addition to the options that do not specify
 debug/release. All builds are considered to be release unless DEBUG=1 is set by
-the user.
-
-Other variables may be defined as needed.
+the user. Other variables may be defined as needed.
 
-Build commands:
+## Build commands:
 
   For each build, a unique build dir and staging dir are created. For example,
-  work/build/mylib/1.0-1adac830f6e and work/staging/mylib/1.0-1adac830f6e.
+  `work/build/mylib/1.0-1adac830f6e` and `work/staging/mylib/1.0-1adac830f6e`.
 
   The following build commands are available for each recipe:
 
-  $(package)_fetch_cmds:
+    $(package)_fetch_cmds:
     Runs from: build dir
     Fetch the source file. If undefined, it will be fetched and verified
     against its hash.
-  $(package)_extract_cmds:
+
+    $(package)_extract_cmds:
     Runs from: build dir
     Verify the source file against its hash and extract it. If undefined, the
     source is assumed to be a tarball.
-  $(package)_preprocess_cmds:
+
+    $(package)_preprocess_cmds:
     Runs from: build dir/$(package)_build_subdir
     Preprocess the source as necessary. If undefined, does nothing.
-  $(package)_config_cmds:
+
+    $(package)_config_cmds:
     Runs from: build dir/$(package)_build_subdir
     Configure the source. If undefined, does nothing.
-  $(package)_build_cmds:
+
+    $(package)_build_cmds:
     Runs from: build dir/$(package)_build_subdir
     Build the source. If undefined, does nothing.
-  $(package)_stage_cmds:
+
+    $(package)_stage_cmds:
     Runs from: build dir/$(package)_build_subdir
     Stage the build results. If undefined, does nothing.
 
   The following variables are available for each recipe:
-  $(1)_staging_dir: package's destination sysroot path
-  $(1)_staging_prefix_dir: prefix path inside of the package's staging dir
-  $(1)_extract_dir: path to the package's extracted sources
-  $(1)_build_dir: path where configure/build/stage commands will be run
-  $(1)_patch_dir: path where the package's patches (if any) are found
+    
+    $(1)_staging_dir: package's destination sysroot path
+    $(1)_staging_prefix_dir: prefix path inside of the package's staging dir
+    $(1)_extract_dir: path to the package's extracted sources
+    $(1)_build_dir: path where configure/build/stage commands will be run
+    $(1)_patch_dir: path where the package's patches (if any) are found
 
 Notes on build commands:
 
@@ -127,4 +143,5 @@ configure step to (usually) correctly configure automatically. Any
 $($(package)_config_opts) will be appended.
 
 Most autotools projects can be properly staged using:
-  $(MAKE) DESTDIR=$($(package)_staging_dir) install
+
+    $(MAKE) DESTDIR=$($(package)_staging_dir) install