Rubocop configuration

Now with a guide on how to setup and use Rubocop, according to the conclusions from the discussion pull request (https://github.com/alphagov/styleguides/pull/36). 
> All cops should be enabled and disabled at the discretion of each project's technical lead.

I'm not sure this is quite correct.  It really should be up to the team or teams responsible for each project.  The technical lead should only be stepping in if there's a failure to reach consensus within the team.