Add API documentation

[maty21]

Hi
The project seems great but it's hard to understand how to use it without an API documentation.

It will be nice if you guys could have a little descriptors about the API

Thanks

[seanpoulter]

Have you two found anything useful for what you're working on? If you've got some of the code fresh in your mind, I'd review a PR that improves any of the documentation. 👍