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.
Is “primitive” a technical term I haven’t come across, or are there words missing here? Might “[backing service](http://12factor.net/backing-services)” be clearer?