Add README explaining branch deprecation

Some external talks mentioned that we had experimental LLVM support
under the android-{4.4,4.9}-llvm branches.  Add a README clarifying that
these branches are obsolete and we've moved all development into the
main android-4.x branches.

Change-Id: I34361dd626c0768eb8c9c430256c00ff1572582e
Signed-off-by: Greg Hackmann <ghackmann@google.com>
1 file changed