Add GOV.UK README guidance

Add a first cut at guidance on how to structure a README for a GOV.UK public project, including examples and justification.

This was produced with @alext, @annashipman, @dcarley. @dsingleton, @evilstreak and @jamiecobbett.
I’m not sure I understand what this means. Would an example of this be “This is a Ruby on Rails app. It uses Bundler to manage its gems.”?