Improve README

This improves the README inline with the [GOV.UK README guidance](https://github.com/alphagov/styleguides/blob/a56ac875b9031eb82bffa9e20b9f19b60aaf4377/use-of-READMEs.md).
Primarily it adds:

- A better intro
- Explicit instructions about how it's Cucumber, and how to run it
- Syntax highlighting
- A description of Nagios integration