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.
> but it famously offers nothing more.

I'm not so sure about this part, I don't want it to sound like we're trashing one of our dependencies.