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

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

> @ftynse Could you take another look? I’ll leave this open until next week - if there are no further comments, I plan to land it around Tuesday.

I haven't heard back from Alex, but I believe all comments have been addressed. I'll go ahead and land this now and, if needed, iterate on it in-tree.

Thanks again, everyone, for the reviews! 🙏🏻

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