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

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

I've added Kristof as a reviewer. Kristof added the existing note on using GitHub's `Co-authored-by`:
* https://github.com/llvm/llvm-project/blob/cd3acd1bff02d0100cbe74307f29c00a3874bc41/llvm/docs/DeveloperPolicy.rst?plain=1#L409

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