add changelog entry for [JENKINS-13185]

changelog.html

@@ -55,6 +55,9 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=bug>
+    Computer.getHostName() returns null when it is not.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-13185">issue 13185</a>)  
   <li class=rfe>
     Improved the page loading performance, especially on large latency network
 </ul>