Small README tweak, add Built in Adapters section

Hi All,

Spent a wee while today trying to get the JSON generated work properly for Ember.

Eventually (after trying to set `config.root`, instantiating my own Serializers in my controllers, etc. etc.) I figured out that the intent is to use JsonApiAdapter :smile: 

So I added a small section to the README just to correct that default, and I additionally added a section on the different adapters for the next person.

Hope it's OK - let me know if you want tweaks.

Cheers,

Nik