docs: Add instructions for how to submit a merge request

Reviewers: hansw, hans

Reviewed By: hans

Subscribers: hans, llvm-commits

Differential Revision: https://reviews.llvm.org/D37936

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313608 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed