[Proposal] move some documentation back to readme

I felt like the homepage readme was a little empty without having some documentation, and it seems like on the Blacklight email list, the wiki pages can be hard to find and navigate. Adopters will often comment about having difficulty finding things on the wiki and I wanted to make it as easy as possible for someone to get going.

Adding some documentation back to main Readme and linking through to the various documentation pages.

Thoughts?