document popolo data model

Pages to document how to use the data model by using some common examples.

Also a tutorial on using the API and a page listing some of the client libraries available.
This differs from the docs at http://popit.poplus.org/docs/api/writing/, which claim "Updating a document is done by sending the whole document in the PUT request. Any fields that are omitted in the request will be removed, and replaced with the default."