This task upstream contains useful background: Document one or two reasonable workflows for Github repositories
Upstream report: https://secure.phabricator.com/T589 (Closed/Declined, upstream will not maintain third-party importers).
A GitHub->Phabricator bridge (similar to T37497 for GitHub->Gerrit) would allow new contributors to make a change without installing arc.
Basically, new GitHub pull requests would get copied into Differential (the Phabricator code review tool).
See prior discussion at T127: Find way to use Differential with plain git (i.e.: without requiring arc).