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.
Pointing at bad examples makes me rather uncomfortable. I like the idea of pointing at examples to follow, but I worry about the cultural effects of pointing out existing code as an example of what *not* to do (and, by extension, potentially shaming those involved). I can see there’s a benefit to demonstrating the point with examples, but in this case I don’t think it’s worth the cost.