Add documentation

This fixes #15 
:+1: I did forget bind. 

About the parameters and extracting a method: I think binding should be enough here as the Promise from above should resolve in the second argument which is the options object for the `count` method. Only specific properties from this object are used and these should not be included in the resolved Promise. 

Apart from that: I will nevertheless extract a method. The reason why I didn't pull it out was that I wanted to show that you don't always have to. As there are other examples with the usage of `bind` and directly defined functions it is not necessary here.