Mention gr as a third party backend

[gr](http://gr-framework.org/) seems to be a graphics framework by @jheinen that, in particular, can be used as a high performance matplotlib backend.  In https://github.com/jheinen/gr/issues/16 @tacaswell suggested (>1y ago) that it be added to matplotlib's documentation, but the issue seems to have fallen through the cracks, so I'm reposting it on this tracker.