Change README to use alphagov readme styleguide

- The mainstay of this pull request is actually captured in the creation of a wiki: https://github.com/alphagov/smartdown/wiki

I've broken the readme down into a much smaller document, that covers an overview of what smartdown is and  provides an example of smartdown in action. After that there's a quick section on dependencies and how to run smartdown from within the GOV.UK VM.

Each new section in the wiki also has a smartdown, html and rendered example, hopefully ensuring that nobody is left too stranded when they have the docs in front of them.