Add user-defined literals to release notes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152424 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.html

@@ -124,6 +124,7 @@ following are now considered to be of production quality:
   <li>Lambda expressions</li>
   <li>Generalized initializers</li>
   <li>Unrestricted unions</li>
+  <li>User-defined literals</li>
 </ul>
 
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