Add mention of ARM atomic builtin support

llvm-svn: 102103

llvm/docs/ReleaseNotes.html

@@ -745,6 +745,10 @@ href="http://blog.llvm.org/2010/04/arm-advanced-simd-neon-intrinsics-and.html">
 <li>The ARM backend now has good support for ARMv4 targets and has been tested
     on StrongARM hardware.  Previously, LLVM only supported ARMv4T and
     newer chips.</li>
+
+<li>Atomic builtins are now supported for ARMv6 and ARMv7 (__sync_synchronize,
+    __sync_fetch_and_add, etc.).</li>
+
 </ul>