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.
This reads a bit strangely. What about `Benefits of using this template`? Then change the items below from `Is` to `it is`