Add recipes for migrating downstream branches of git mirrors

Add some common recipes for downstream users developing on top of the
existing git mirrors. These instructions show how to migrate local
branches to the monorepo.

Differential Revision: https://reviews.llvm.org/D56550



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353713 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed