Tweaks to service.gov.uk subdomain guidance

I've generally cleaned this up a bit, attempted to make parts of it more
readable, reordered sections to eliminate the need to reference requirements
that haven't yet been described, and added a few technical links and examples
where we had said we would provide them later.
Merge pull request #63 from alphagov/servicegovuk-tweaks

Tweaks to service.gov.uk subdomain guidance