Update the README.md

The readme has been brought up to date with the govuk styleguide at
https://github.com/alphagov/styleguides/blob/master/use-of-READMEs.md\

https://www.pivotaltracker.com/story/show/84366924
I think it's necessary to set up the `published_documents` exchange, and the appropriate rabbitmq user and permissions, in order for this to work.  These are set up by puppet on the development boxes, but the README should at least have a note that this is necessary (and ideally have details of what the setup should be).