Add mergely.com to real-world uses

index.html

@@ -96,6 +96,7 @@
       <li><a href="http://buzzard.ups.edu/">Sage</a> demo (math system)</li>
       <li><a href="http://www.sourcelair.com/">sourceLair</a> (online IDE)</li>
       <li><a href="http://eloquentjavascript.net/chapter1.html">Eloquent JavaScript</a> (book)</a></li>
+      <li><a href="http://www.mergely.com/">Mergely</a> (interactive diffing)</li>
      </ul>
 
   </div></div>