Better Readme and Docs

Mainly adds a features section to the new Readme but also does some minor edits and adds a section about updating to the docs.

I liked the separation of the Readme but I thought that one of the things it lost was the "wow Spacemacs is cool" factor. Since one of the roles of the Readme is as Marketing for using a Github project, I thought it would be good to add an overview of the best parts of Spacemacs. I'm not sure I would have started using Spacemacs if I hadn't had the Readme with all the amazing features thrown in my face.

The features section also contains extensive links into the documentation to make it more discoverable. One of my biggest worries with the current Readme is that new users won't find the amazing documentation because it is only mentioned in a few places like a tiny link in the upper right.