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.
We should make gitter more prominent. If i did not know what gitter was, I'd think it was part of our build / CI stack.