Replace ‘Future Plans’ with a mention of Chris Wanstrath’s hub

I have left the future plans in the README so long that they are no
longer relevant, and I have no intention to address them any time
soon.  So instead I am replacing that part of the README with a link
to the great ‘hub’ program since it offers great features for
interacting with GitHub repositories, just as this project attempts,
although not nearly on the same scale as ‘hub’.