README: Document passing friendship to callbacks

Documentation doesn't mention how to access the friend being followed/unfollowed from `after` callbacks. I document my adventures on how I discovered this [here](https://gist.github.com/ericboehs/ccc6b78349fc32e5f1bb).

By the way, the `value` is returned when creating the friendship. Therefore, `value` will be `nil` for before callbacks. This is because it is being called *before* the friendship code is ever ran.