Mention atomics support in the release notes.
Thanks to Nico Weber for the suggestion.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154648 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/ReleaseNotes.html b/docs/ReleaseNotes.html
index 7ac6b69..d05c4b6 100644
--- a/docs/ReleaseNotes.html
+++ b/docs/ReleaseNotes.html
@@ -132,6 +132,8 @@
   <li>Unrestricted unions</li>
   <li>User-defined literals</li>
   <li>Forward-declared enumerations</li>
+  <li>Atomics (both libc++'s and libstdc++4.7's <tt>&lt;atomic&gt;</tt> are
+      supported)</li>
 </ul>
 
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