Stub out a release notes section on -fcatch-undefined-behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162636 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.html

@@ -170,6 +170,13 @@ int f(vector<map<int, double>>);
 
   </li>
 
+  <li>Clang's <tt>-fcatch-undefined-behavior</tt> option has grown the ability
+  to check for several new types of undefined behavior.
+
+  <!-- Flesh this out prior to release. -->
+
+  </li>
+
 </ul>
 
 <h4 id="tlsmodel">Support for <code>tls_model</code> attribute</h4>