Can we have an API reference?

As well as the general readme, I think reflux should have an API.md file that would list the properties of the definition objects that you pass into createStore, createAction etc. as well as their methods.

At the moment, even Store#getInitialState isn't even mentioned in the readme, which is very confusing. I would be happy to make a PR, but I'm sure someone with a better knowledge of the codebase would make a better API reference.