update changelog for new mime type video/webm

changelog.html

@@ -74,6 +74,8 @@ Upcoming changes</a>
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-12397">issue 12397</a>)   
   <li class=rfe>
     Jenkins no longer failes to start if plugins have cyclic dependencies, but deactivates these plugins.
+  <li class=rfe>
+    Adding the video/webm mime type for files with extension .webm.
 </ul>
 </div><!--=TRUNK-END=-->