added tip about mapping

Author: lukas-vlcek

help/index.html

@@ -69,6 +69,7 @@ <h4>Tips</h4>
 rather than <code>elasticsearch.mydomain.com</code> - it makes it easier for
 other users to run your example, without having to edit it first.
 </li>
+<li>It is always better to include your <a href="../guide/reference/mapping/">mapping</a> in the example.</li>
 </ul>
 <h4>Other APIs</h4>
 <p>