Add more documentation for Android Clang/LLVM toolchain.

This adds info about where to find the latest version of these docs on
the web, along with our public mailing list. It also explains how we
pick our SVN revisions to rebase from.

Change-Id: Ic2d1c7ce617b273f1bc839d9d7bfe3482cf89459
(cherry picked from commit 1c9d8ed98e30ce8226fccae42582eb586ff8fce5)
3 files changed