Moving to GitHub - Unified Proposal

This document describes the proposal to move to GitHub, and
compare the two proposals through various workflow examples,
presenting the current set of commands following by the ones
involved in each of the two proposals.

It is intended to supersede the previous "submodule proposal"
document entirely, and drive the discussion at the BoF during
the next Dev Meeting.

Differential Revision: https://reviews.llvm.org/D24167

llvm-svn: 284077