[TAV-441] Detailed Readme.

Installation instructions; explanation of overriding views, controllers and
models; details of callbacks and renderers; etc.
Fix heading levels.

Most were one level too-low.
Add information on the workers to the Readme.
Merge branch 'rh/docs'

* rh/docs:
  Add information on the workers to the Readme.
  Fix heading levels.
  [TAV-441] Detailed Readme.