Update README

Given that we now have [README guidelines](/alphagov/styleguides/blob/update-readme/use-of-READMEs.md), it makes sense to update the README to adhere to them. Most points are not relevant as this is not an app, but worth adhering to those that are.

I have also tried to make it clearer as it took me ages to find the Git commit guidelines. Comments on clarify and usefulness of new README welcome.

I am not sure about the typographic styles and colours. Are these the correct, up-to-date links and are they maintained?

I've also updated the one-line description of this repo, per the README guidelines.