Add docs for the context processors

[fredkingham]

we use (mkdocs)[http://www.mkdocs.org/] to create the docs for http://opal.openhealthcare.org.uk/

we really should have the context processors in http://opal.openhealthcare.org.uk/docs/reference/reference_guides/

• [x] install and run mkdocs locally (cd opal/doc; mkdocs serve)
• [ ] add context_processors.md
• [ ] add your new file to opal/doc/docs/reference/reference_guides.md and opal/doc/mkdocs.yml
• [ ] add markdown in context_processors similar to the desciption in the change log

[davidmiller]

I think we might like to include in the criteria for this pushing them live once it's been merged?

(In the process you might see the tiny bit of ruby code we use...)