[docs] GitHub Proposal for LLVM

This document was crafted from the various (320+) emails between 2nd June and
20th July regarding the move to GitHub. It tried to consolidate every issue that
was raised and every solution that was presented to have a GitHub repository
with sub-modules.

It *does not* try to argue whether sub-modules are better or worse than any other
Git solution, nor if Git is better than any other VCS, nor if GitHub is better
than any other free code hosting service. This is just the final conclusions of
48 days and 320 emails (plus a lot of IRC discussions) on the LLVM community.

This document will be presented at the survey that the foundation will setup for
us to decide if we move to this solution or not. It reflects what was discussed
on the lists, but it's not authoritative. If something is not clear enough,
please refer to the mailing list discussions (hint: search for "GitHub").

Review: https://reviews.llvm.org/D22463
llvm-svn: 276097