[Doc] Update OS X build notes for 10.11 SDK

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+Deterministic OS X Dmg Notes.
+
+Working OS X DMGs are created in Linux by combining a recent clang,
+the Apple binutils (ld, ar, etc) and DMG authoring tools.
+
+Apple uses clang extensively for development and has upstreamed the necessary
+functionality so that a vanilla clang can take advantage. It supports the use
+of -F, -target, -mmacosx-version-min, and --sysroot, which are all necessary
+when building for OS X.
+
+Apple's version of binutils (called cctools) contains lots of functionality
+missing in the FSF's binutils. In addition to extra linker options for
+frameworks and sysroots, several other tools are needed as well such as
+install_name_tool, lipo, and nmedit. These do not build under linux, so they
+have been patched to do so. The work here was used as a starting point:
+[mingwandroid/toolchain4](https://github.com/mingwandroid/toolchain4).
+
+In order to build a working toolchain, the following source packages are needed
+from Apple: cctools, dyld, and ld64.
+
+These tools inject timestamps by default, which produce non-deterministic
+binaries. The ZERO_AR_DATE environment variable is used to disable that.
+
+This version of cctools has been patched to use the current version of clang's
+headers and and its libLTO.so rather than those from llvmgcc, as it was
+originally done in toolchain4.
+
+To complicate things further, all builds must target an Apple SDK. These SDKs
+are free to download, but not redistributable.
+To obtain it, register for a developer account, then download the [Xcode 7.3.1 dmg](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_7.3.1/Xcode_7.3.1.dmg).
+
+This file is several gigabytes in size, but only a single directory inside is
+needed:
+```
+Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk
+```
+
+Unfortunately, the usual linux tools (7zip, hpmount, loopback mount) are incapable of opening this file.
+To create a tarball suitable for Gitian input, mount the dmg in OS X, then create it with:
+```
+  $ tar -C /Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/ -czf MacOSX10.11.sdk.tar.gz MacOSX10.11.sdk
+```
+
+The Gitian descriptors build 2 sets of files: Linux tools, then Apple binaries
+which are created using these tools. The build process has been designed to
+avoid including the SDK's files in Gitian's outputs. All interim tarballs are
+fully deterministic and may be freely redistributed.
+
+genisoimage is used to create the initial DMG. It is not deterministic as-is,
+so it has been patched. A system genisoimage will work fine, but it will not
+be deterministic because the file-order will change between invocations.
+The patch can be seen here:  [theuni/osx-cross-depends](https://raw.githubusercontent.com/theuni/osx-cross-depends/master/patches/cdrtools/genisoimage.diff).
+No effort was made to fix this cleanly, so it likely leaks memory badly. But
+it's only used for a single invocation, so that's no real concern.
+
+genisoimage cannot compress DMGs, so afterwards, the 'dmg' tool from the
+libdmg-hfsplus project is used to compress it. There are several bugs in this
+tool and its maintainer has seemingly abandoned the project. It has been forked
+and is available (with fixes) here: [theuni/libdmg-hfsplus](https://github.com/theuni/libdmg-hfsplus).
+
+The 'dmg' tool has the ability to create DMGs from scratch as well, but this
+functionality is broken. Only the compression feature is currently used.
+Ideally, the creation could be fixed and genisoimage would no longer be necessary.
+
+Background images and other features can be added to DMG files by inserting a
+.DS_Store before creation. This is generated by the script
+contrib/macdeploy/custom_dsstore.py.
+
+As of OS X Mavericks (10.9), using an Apple-blessed key to sign binaries is a
+requirement in order to satisfy the new Gatekeeper requirements. Because this
+private key cannot be shared, we'll have to be a bit creative in order for the
+build process to remain somewhat deterministic. Here's how it works:
+
+- Builders use Gitian to create an unsigned release. This outputs an unsigned
+  dmg which users may choose to bless and run. It also outputs an unsigned app
+  structure in the form of a tarball, which also contains all of the tools
+  that have been previously (deterministically) built in order to create a
+  final dmg.
+- The Apple keyholder uses this unsigned app to create a detached signature,
+  using the script that is also included there. Detached signatures are available from this [repository](https://github.com/bitcoin-core/bitcoin-detached-sigs).
+- Builders feed the unsigned app + detached signature back into Gitian. It
+  uses the pre-built tools to recombine the pieces into a deterministic dmg.