Demos: Link widget demo to learn site

Ref jquery/jqueryui.com#39
Closes jquerygh-1637

Author: jzaefferer

demos/widget/default.html

@@ -166,6 +166,7 @@
 <div class="demo-description">
 <p>This demo shows a simple custom widget built using the widget factory (jquery.ui.widget.js).</p>
 <p>The three boxes are initialized in different ways. Clicking them changes their background color. View source to see how it works, its heavily commented</p>
+<p>To learn more about the widget factory, <a href="http://learn.jquery.com/jquery-ui/widget-factory/">visit learn.jquery.com</a>.</p>
 </div>
 </body>
 </html>