documentation: combining object reference and class attribute

REFERENCE.md

@@ -622,6 +622,13 @@ is compiled to:
       Hello!
     </div>
 
+The `:class` attribute may be used in conjunction with an object
+reference.  The compiled element will have the union of all classes.
+
+    - user = User.find(1)
+    %p[user]{:class => 'alpha bravo'}
+    <p id="user_1" class="alpha bravo user"></p>
+
 ## Doctype: `!!!`
 
 When describing HTML documents with Haml, you can have a document type or XML