Add project collaborator username completion

Hi,
this commit addresses feature request in #1.
Implementation is done per your suggestion in that thread.

I did a minimal update to the docs.
I wanted to do the same thing for readme, but after agonizing over the wording for 20 minutes I gave up. I think you can do the update faster and just the way you want. If you still want me to make an update please, let me know.

I hope everything else is good!