[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.
----------------
banach-space wrote:

Thanks for raising this! Admittedly, I hadn’t really considered the employer-employee relationship in this context. That said, should LLVM’s Code Review policies (or any other Open Source project) _mandate, guide, or influence_ what that relationship should be?  

Let me clarify where I was coming from when writing this. My focus was on cases where some code exists in an Open Source repository, and a _community volunteer_ decides to _move_ that code from the downstream project to LLVM - either _with or without_ additional changes. To me, it seems natural that the original author should be consulted, and that any changes should be properly attributed in such cases.

Regarding your point about employer-employee relationships, I’m not sure what we could add to make this clearer, as it seems highly context-dependent. And very different to what I originally had in mind. Are there any specific **licensing** and/or **copyright** issues that we should consider? I’d hope that the existing LLVM [Developer Policy](https://llvm.org/docs/DeveloperPolicy.html#copyright-license-and-patents) is sufficient here.

Ping @kbeyls , who has spent a considerable amount of time on LLVM's relicensing and is my go-to expert this kind of things.

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