Add documentation for methods/events

It would be really cool to have the documentation on each method/event that we can use. We can look into README.md for that, but I think it'll be way more better if we had the separate page that describes each API method/event that we can subscribe to, what arguments it accepts, what does it return, how it works, which errors can it throw etc.

This could be implemented by using the Github Wiki, or adding a Markdown file to a repository (IMHO the second way would be more cool).

I can make a PR on that, just wondering if it would be useful.

Start with README.md and inventory the methods and events exposed by the API. Done means adding a separate Markdown reference that documents each method/event, its arguments, return value, behavior, and possible errors.

Written by the indexing model from the issue text.