Note x86 regmask operands in release notes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150712 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/ReleaseNotes.html b/docs/ReleaseNotes.html
index c284b9c..364ac9e 100644
--- a/docs/ReleaseNotes.html
+++ b/docs/ReleaseNotes.html
@@ -377,7 +377,9 @@
 <ul>
   <li>Bug fixes and improved support for AVX1</li>
   <li>Support for AVX2 (still incomplete at this point)</li>
-  <li>....</li>
+  <li>Call instructions use the new register mask operands for faster compile
+  times and better support for different calling conventions.  The old WINCALL
+  instructions are no longer needed.</li>
 </ul>
 
 </div>