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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

Tentatively LGTM.

Please wait for someone else to approve though and comment on how they want to deal with documenting current state versus making reccomendations.

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