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.

Also introduce a `gov.uk` directory to make it clear that this guidance only applies to GOV.UK.

This was produced with @alext, @annashipman, @dcarley. @dsingleton, @evilstreak and @jamiecobbett.
Updated in cfddf07