Adding commit msg guidelines to dev policy

Adding commit msg guidelines to dev policy

After much bike shed discussions, we seem to agree to a few loose
but relevant guidelines on how to prepare a commit message. It also
points the attribution section to the new commit messages section
to deduplicate information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232334 91177308-0d34-0410-b5e6-96231b3b80d8