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

[Andrzej Warzyński via llvm-commits]

================
@@ -272,3 +272,19 @@ Developers should participate in code reviews as both reviewers and
 authors. If someone is kind enough to review your code, you should return the
 favor for someone else. Note that anyone is welcome to review and give feedback
 on a patch, but approval of patches should be consistent with the policy above.
+
+Upstreaming Changes to LLVM
+===========================
+
+When upstreaming your own changes from a downstream project to LLVM, simply
+follow the process outlined above.
+
+When upstreaming changes originally written by someone else:  
+
+* Ensure that the original author(s) are aware of and approve the upstreaming
+  of their code, and that they are comfortable with you leading the process.
+* Properly attribute the original changes, e.g., by creating a commit with
+  multiple authors ([GitHub guide](https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors)).  
+* Invite the original author(s) to review the changes, and also include
+  additional reviewers. Specifically, an LGTM from a (co-)author does not
+  constitute approval to land a change.
----------------
banach-space wrote:

Thank you for suggesting this. Let me incorporate it.

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