Update discussion documentation

As more people are having to work on discussion, I thought it might be a good idea to update the documentation.

I have a fix idea for the [last bit](https://github.com/guardian/frontend/compare/discussion-documentation?expand=1#diff-318713d77978651104857a733e567a6bR42), so hopefully this be rectified soon.

![Yap yap yap](http://www.abload.de/img/lilly3gsunb.gif)