Proposal: accept pull requests on GitHub

[Richard Eisenberg]

> > (I'm tempted naively to ask: is there an automated way to go from a GitHub PR to a Phab ticket?  Then we could convert the former (if someone wants to submit that way) into the latter.)

Or: is there a way contributors can create a Phab differential off a GitHub branch? This bypasses the GitHub PR but still provides a similar ease-of-use. This one seems rather easy to imagine.

Richard