Add a note about using "Differential Revision:" in commit messages

I noticed this convention from the commit logs.  It seems like it
would be useful to document it, to encourage other committers to link
back to code reviews in their commits.

Differential Revision: http://llvm-reviews.chandlerc.com/D2678

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201160 91177308-0d34-0410-b5e6-96231b3b80d8