Documentation: broken link to Gitter

[brinklec]

Main Github page has link to quiver Gitter at bottom. I'd love to see a Gitter page for this, but currently the link yields 404 Not Found.

[rossabaker]

I'll look into opening a room when we move this project to getnelson. In the meantime, many of the quiver developers are on getnelson/nelson Gitter.