Write documentation

[mgax]

This is a running list of things to document:

• Using the package
  • [ ] Configure a cache to take advantage of the audience/segment caching.
  • [x] Be sure to use |newsletter_richtext template filter to generate full URLs.
  • [x] How to embed images with full URLs. (But beware of using image renditions in newsletters - make sure you don't delete the renditions later or it will break the emails.)
  • [ ] How to define email-only and web-only content blocks.
  • [ ] List of available settings.
• Customising
  • [ ] Render templates with django-mjml
  • [x] Render email templates using plain html.
  • [x] Custom recipients model.
  • [ ] Writing/extending a campaign backend.