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 think it's a file line - the "Full documentation of the API" wording is fairly unclear in itself, but is intended to convey a general feel for what too much documentation is. Bad examples provide context for that.

I agree that we shouldn't be shaming here, but it's hard to prove the point without examples. Do you have a suggested alternative that still fulfils the intent?