Add to contributors in the current PR

[flanakin]

Is your feature request related to a problem? Please describe.
There are times when someone starts a PR, but never finishes it. In those cases, I want to add them upfront so I don't forget later, but I don't want to commit a second PR when their contribution isn't complete.

Describe the solution you'd like
Add a commit to the current PR rather than creating a new PR.

Describe alternatives you've considered
Manually add them :-(