[PATCH] Add commit message guidelines to developer policy

[Sanjoy Das]

I tend to add a prefix like `[IRCE]` or `[SCEV]` to the subject of my commit messages to quickly establish context.  I've seen others do it too.  Does it make sense to codify that practice as a suggestion in this document?


REPOSITORY
  rL LLVM

http://reviews.llvm.org/D8197

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