Documenter.jl docs

[zsunberg]

It would be good to move the docs currently in the README to Documenter.jl. I haven't set it up yet though.

This is a good guide for how to think about documentation: https://documentation.divio.com/

[zsunberg]

Some things that should go in the docs:

• difference between state and observation (Replace step! with act! and observe #26 (review))

[zsunberg]

#42 adds basic docs, but things like the checkmark above still need to be added