Add documentation

[tawalaya]

I was tiring to invoke a function using the this lib. I had to jump between the wsk client code and this to figure out what to do here. For instance here it would have been really helpful to know that payload should be a map[string]interface{} to send data.

[rabbah]

Thanks @tawalaya - I agree. If you are inclined to add documentation and send a PR, we'd be happy to accept it. You don't need a CLA for documentation changes.

I think the client can benefit from some refactoring also but that's a separate topic.