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

[Simon Tatham via llvm-commits]

https://github.com/statham-arm approved this pull request.

LGTM – this is definitely an improvement on the docs before this commit, and also an improvement on the first version I saw in this review.

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