[llvm] [llvm][docs] Add notes on upstreaming code from downstream projects (PR #129743)

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

> FWIW, this LGTM (with minor comment).

Updated. Could you approve it using the GitHub UI? Once that’s done, I’ll post an update on Discourse and invite more reviewers if anyone wishes to comment.

https://github.com/llvm/llvm-project/pull/129743