Added some documentation to alert_box

Added documentation to `AlertBoxHelper` explaining that passing HTML should be accomplished by using a block rather that an argument. If a `link_to` is passed as a regular argument it wont add `alert-link` class to the link element.
What I found out is that Middleman (with Padrino) uses vanilla ERB, and not Rails ERB, so when you pass a block with `do`, you don't have to use `<%=` but `<%`.

That's why it was failing for me