Also, since the master branch is being managed external to GitHub, I'd suggest protecting that branch and making a new one (e.g. github) that can be used to quickly merge the PRs to. This will allow simple use of the GitHub web interface to merge PRs using squash / rebase / merge commits to that branch from which you can move those commits to master in your external repository. You could do this with patch sets or setup github as a remote or some other preferred way.
The idea behind this workflow is that contributors get invited after their first contribution and from that point on can help with merging new PRs into the github branch but would not be able to change the master branch (as that's hosted external to GitHub and protected). This way you can get a pool of contributors helping out with simple changes and give the developers a single source to find the changes that have been community approved.