Simplify intro text and provide a very simple example

- Thought it would be good to show any new users what the outcome of using the DSL would be
- Shows an image of the final outcome within a GOV.UK sample page
- Will break into more complex concepts around logic as we go.
*NB:* My intention is to move the majority of the 'node' logic examples to the [wiki](https://github.com/alphagov/smartdown/wiki) so this readme just provides an intro, installation, test and running guides. All further info around specific tasks can sit in the wiki.