Improve ReadMe content

[m-triassi]

Summary

Closes #12

Adds a more decriptive and useful readme section.
Outlines how to leverage Contentable and Renderable models, as well as how to dress them with layouts.

[kfriars]

Nice docs improvements. I think ripping Layoutable stuff out of this package and into its own makes sense, as Layoutables will hopefully be replaced by something better eventually.

[m-triassi]

Sure, as i was writing the docs it definitely felt like they could exist separately. Lets talk about our plan there later!