[WIP] Add a basic description of special ERB tags

This is my very first pull request!

Should address the issue: #25, but is not quite ready to go yet. 
Feel free to review :)

Done during the [Codebar.io 24pullrequests special](http://codebar.io/events/24-pull-requests-2014) today.

With help from @fanaugen

the `<p>` tag should probably start above :)