Improve documentation

[thill-odi]

The existing documentation is good as far as it goes, but leaves out a lot of context. To illustrate more fully how the library is to be used, we need to make clear that the library is essentially used to create a (very simple) API.

This means we need to lay out a straightforward sequence for developers walking them through from receiving the request to returning the response. We thus need steps, with examples, illustrating:

1. Receiving a request
2. Parsing the request for parameters
3. Querying the database
4. Using the query results to create data items
5. Sequencing these data items
6. Wrapping up these data items as an RPDE feed
7. Serialising the RPDE feed to JSON
8. Returning this JSON as a response