299 doc restructure

Here's my first stab at this.

The idea is to trim the main README down to "everything you need to know in 5 minutes", and then break out the rest into topic documents.
Add a paragraph that says something like.

See [the documentation](https://github.com/RESTful-Drupal/restful/blob/7.x-1.x/plugins/restful/RestfulEntityBase.php#L14-L71) to know more about the field mapping. This mapping configuration will be used to get the output for your public field just the way you need. There are lots of options to get it right in a reusable way, but if you need even more flexibility just provide your custom `callback`.