Add docstrings, API docs

[briancurtin]

First off, I'm really liking the library, so thanks for putting out there!

As I'm using it, I'm finding it a little hard to navigate because there aren't docs or docstrings, so I'm looking through the source and tests to figure out how a lot of things work and sort of guessing at things.

Would you accept PRs if I added docs to this project?

[cybergrind]

hi @briancurtin! sure, we will accept such PR's

[briancurtin]

Ok cool! Once I get through more of what I'm working on I'll put some work in. Would you accept type annotations as well or should I leave those out?

[cybergrind]

@briancurtin we still have 3.5 support so not all type annotations are supported in it. but if you put them in docstrings it would be great