Expand documentation

[zanderle]

I've found this add-on to be very helpful, but often the docs fall a bit short (especially for a newbie like me). For example 'Extending the adapter/serializer' - it's awesome to have that chapter, but it would be even better if it showed what an example extension might look like.
But that's just one example.

I'd love to help out, I'm just not sure I'm familiar enough with the project to do so.

[zanderle]

Somewhat related to #125 I think