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
rummager is a dependency because search-admin sends post requests to update rummager whenever things change.  So I think the descriptive text here should be something like "search-admin sends updates to rummager when best-bets are edited".