Data types documentation

Addresses issue #100 
I tried to come up with a reasonable example, would love feedback and suggestions if you have any!

You can see the rendered markdown in my branch [here](https://github.com/mmacaula/ampersand-state/tree/dataTypes-documentation)

Before the "For example" this would be a great place to put the 4 functions in a list defining what they're passed, and what their function is (i.e. what they should return);