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

1 files changed, 53 insertions, 0 deletions

diff --git a/depends/description.md b/depends/description.md
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+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
+
+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
+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
+
+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.
+
+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.
+
+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,
+etc), and as well as a hash of the same data for each recursive dependency. If
+any portion of a package's build recipe changes, it will be rebuilt as well as
+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.
+
+Each package is configured and patched so that it will yield the same
+build-results with each consequent build, within a reasonable set of
+constraints. Some things like timestamp insertion are unavoidable, and are
+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
+
+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
+
+Build and staging dirs are wiped after use, and any previous version of a
+cached result is removed following a successful build. Automated builders
+should be able to build each revision and store the results with no further
+intervention.