Say something about -vectorize in the release notes.

llvm-svn: 154788

llvm/docs/ReleaseNotes.html

@@ -311,6 +311,9 @@ Release Notes</a>.</h1>
   <li>The loop unroll pass now is able to unroll loops with run-time trip counts.
       This feature is turned off by default, and is enabled with the
       <code>-unroll-runtime</code> flag.</li>
+  <li>A new basic-block autovectorization pass is available. Pass
+      <code>-vectorize</code> to run this pass along with some associated
+      post-vectorization cleanup passes.</li>
   <li>....</li>
 </ul>