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

[Mark Seaborn]

  Closed by commit rL201160 (authored by @mseaborn).

CHANGED PRIOR TO COMMIT
  http://llvm-reviews.chandlerc.com/D2678?vs=6903&id=6992#toc

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

COMMIT
  http://llvm-reviews.chandlerc.com/rL201160