[llvm] [docs][GitHub] Document alternative approach to stacked PRs (PR #132424)

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

Thanks! I've posted https://discourse.llvm.org/t/rfc-documenting-stacked-pr-practices/ to help broadcast this discussion.

https://github.com/llvm/llvm-project/pull/132424