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

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

This is an attempt to encourage folks to specify which PRs their contributions depend on. Recent example:
* https://github.com/llvm/llvm-project/pull/131048

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