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). 
So, how about

> The GDS-styleguide cops are validated by default and present in the gds-ruby-styleguide.yml configuration. Other validators that may be useful to your project can be found in the three "other-***.yml" files. Choice of cops to use is left to the discretion of your team.