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.

![](http://41.media.tumblr.com/8b5695ccef6d2991415f6f21e36b6fe6/tumblr_ng0vhzGsi91tz7xmqo1_500.jpg)
An aside: I'm not sure what adding the DSL link actually gives us here. I don't feel like my knowledge of the project is particularly enriched by having that link there. (Also: Why MF and not wikipedia? Why DSL and not lexer?) Happy to take a steer on this because my current feeling is deep in the shruggy zone ¯\_(ツ)_/¯