[brianmario]

The Co-authored-by trailer isn't actually new. In fact it was already being used by over ~1M commits on GitHub before we launched the feature! :)

The git-core commit message conventions docs mention it (https://git.wiki.kernel.org/index.php/CommitMessageConventio...) as do the OpenStack commit message convention docs (https://wiki.openstack.org/wiki/GitCommitMessages#Including_...).