message: docs: provide co-author guidance in contributer guide.
  Addresses
diff: @@ -104,6 +104,11 @@ versioning guidelines:
  update this field if your PR diverges during review.
  * Your PR description should have details on what the PR does. If it fixes an existing issue it
  should end with "Fixes #XXX".
  +* If your PR is co-authored or based on an earlier PR from another contributor,
  + please attribute them with `Co-authored-by: name <name@example.com>`. See
  + GitHub's [multiple author
  + guidance](https://help.github.com/en/github/committing-changes-to-your-project/creating-a-commit-with-multiple-authors)
  + for further details.
  * When all of the tests are passing and all other conditions described herein are satisfied, a
  maintainer will be assigned to review and merge the PR.
  * Once you submit a PR, *please do not rebase it*. It's much easier to review if subsequent commits
  