Add note to index.html so people know examples are interactive

Author: P. Scott DeVos

index.html

@@ -56,5 +56,7 @@ <h1>Code Examples</h1>
       </a>
     </li>
   </ul>
+  <h2>Note</h2>
+  <p>Try moving your mouse with the primary button down.</p>
 </body>
 </html>