Update README

[pjfitzgibbons]

I'll try to run my stuff (a game) with brown shoes and see what is missing right now. Which points to the need for a how-to section in the README.

Definitely we need a how-to section in the README. You can totally give your game a try, but it is unlikely that it will work yet. We have only basic implementations of buttons, colors, flows, sounds, stroke, fill, and shapes so far. It is not nearly to the level of functionality that Purple Shoes has (yet!).

But we have also written specifications for all of the behavior we have implemented. And discovered lots of Shoes quirks along the way. Part of the goal is to uncover hidden behaviors and make sure they are sane.

[wasnotrice]

README has instructions for setting up, running specs, and running apps, as of f0ff004