Document installing dependencies

@charlierudolph @allewun what do you think?
Cool! Image has uneven whitespace though. Also, I like to pass all .pngs through [ImageOptim](https://imageoptim.com/):

![screen shot 2014-12-26 at 9 05 05 pm](https://cloud.githubusercontent.com/assets/1256911/5560502/7f32917e-8d43-11e4-8765-4e60aadc85a3.png)