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.
Does this code work for you with Middleman 3? It doesn't work for me, I have to update to Middleman 4, that's why I added the comment below about helpers not (yet) supported in Middleman.

Also, to pass a block, you can use the syntax:

```rhtml
<%= alert_box title: 'Thanks', context: :info do %>
  <%= link_to('Link', '#') %>
<% end %>
```