minor: making some readme improvements

* slimmed down existing readme, made it easily consumable (the "5 min readme")
* added clear sections for compatibility, installation, usage, etc.
* moved mixed tutorial content into the wiki and provided a link
* updated text all over, made consistent with 1.x driver readme
* added a note to show that this branch is not the current release

In short, I'm moving a lot of repetative and redundant content into the wiki and trying to slim down our readme into something thats more readable and easy to consume. I'm also trying to standardize the readmes between our 2.x and 1.x branches.

The diff view is confusing, I recommend reviewing this on Github:
https://github.com/brandonblack/mongo-ruby-driver/blob/readme-2.x/README.md