Shopping-Cart demo: Add new demo to droppable demo index

Author: jzaefferer

demos/droppable/index.html

@@ -15,6 +15,7 @@ <h4>Examples</h4>
 		<li><a href="propagation.html">Prevent propagation</a></li>
 		<li><a href="visual-feedback.html">Visual feedback</a></li>
 		<li><a href="revert.html">Revert draggable position</a></li>
+		<li><a href="shopping-cart.html">Shopping Cart</a></li>
 		<li><a href="photo-manager.html">Simple photo manager</a></li>
 	</ul>
 </div>