Updates readme.

**Still a super rough draft**

Here are some updates to the readme.

The changes:

1. Explains Marionette more succinctly and descriptively (I hope)
2. Removes unnecessary stuff.

Things removed either:

1. Don't belong anywhere
2. Belong on the website

While we might not be able to merge it just yet, I'd like to see us work toward getting something like this in.
Yeah just like:

> ### Backbone.Radio
> **Messaging patterns for Backbone applications.**

> ---
> Use Backbone.Radio to build large, maintainable, and decoupled applications.

> Backbone.Radio is a collection of messaging patterns for Backbone applications. It uses Backbone.Events as a pub-sub message bus, then adds semantics to your communications through the addition of two new messaging patterns, Commands and Requests. The three systems are bound together as Channels, which provide explicit namespacing to your communications.