[css-cascade] Update changes list.

Author: fantasai

css-cascade/Overview.bs

@@ -851,6 +851,7 @@ Changes Since the 3 October 2013 Candidate Recommendation</h3>
 
 	<ul>
 		<li>Defined <a>environment encoding</a> of imported style sheets.
+		<li>Referenced [[!CSS3SYN]] for syntax of ''!important'' rules.
 	</ul>
 
 	<p>A <a href="http://dev.w3.org/csswg/css-cascade/issues-cr-2013">Disposition of Comments</a> is available.