added documentation for anttasks.properties

docs/anttask.html

@@ -26,6 +26,10 @@ This task is included in the checkstyle distribution.</p>
   &lt;taskdef name=&quot;checkstyle&quot;
            classname=&quot;com.puppycrawl.tools.checkstyle.CheckStyleTask&quot;/&gt;
 </pre>
+<p>Alternatively, since checkstyle version 2.1 you can use <code>taskdef</code>'s resource attribute:</p>
+<pre>
+  &lt;taskdef resource=&quot;anttasks.properties&quot;/&gt;
+</pre>
 
 
 <h3>Parameters</h3>