Document use of Co-author-by git tag.

[kristof.beyls]

Summary

We are already using the Co-author-by git tag, but don't have documentation in
our developer policy about it. Fix that.

[lattner]

Thank you for clarifying this!

[danielkiss]

llvm/docs/DeveloperPolicy.rst

270

Thanks!
maybe a link to the manual instead?
https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors

Link with an anchor to the section is maybe too much.
https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors#creating-co-authored-commits-on-the-command-line

[kristof.beyls]

llvm/docs/DeveloperPolicy.rst

270

I've wondered what exactly to link to.
Rather than pointing to a manual on how to add those tags in a github-specific way, I wanted to point to something that explains why co-author-by tags are useful and what value they bring. The github blog post I'm pointing to in the link is the best I've found (after searching for maybe 5 minutes).
So, I think the link to the github blog is slightly better than a link to the github-specific manual page. That being said, I wouldn't be surprised if there is a better page to point to - it's just that I haven't found it.

[arichardson]

llvm/docs/DeveloperPolicy.rst

269

Shouldn't this be Co-authored-by? At least that's the form I've seen and used so far.

[kristof.beyls]

llvm/docs/DeveloperPolicy.rst

269

Thanks for noticing!
I've now fixed this in cdf03f763484

llvm/docs/DeveloperPolicy.rst

 * If you're not the original author, ensure the 'Author' property of the commit is
   set to the original author and the 'Committer' property is set to yourself.
   You can use a command similar to
   ``git commit --amend --author="John Doe <[e-mail]>"`` to correct the
   author property if it is incorrect. See `Attribution of Changes`_ for more
   information including the method we used for attribution before the project
   migrated to git.
 
+  In the rare situation where there are multiple authors, please use the `git
+  tag 'Co-author-by:' to list the additional authors
+  <https://github.blog/2018-01-29-commit-together-with-co-authors/>`_.
+
 * The title should be concise. Because all commits are emailed to the list with
   the first line as the subject, long titles are frowned upon.  Short titles
   also look better in `git log`.
 
 * When the changes are restricted to a specific part of the code (e.g. a
   back-end or optimization pass), it is customary to add a tag to the
   beginning of the line in square brackets.  For example, "[SCEV] ..."
   or "[OpenMP] ...". This helps email filters and searches for post-commit