Update the release notes about the status of the vectorizers.

llvm-svn: 187714
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index e776dce..00c848f 100644
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -62,6 +62,11 @@
   or later instead. For more information, see the `Getting Started using Visual
   Studio <GettingStartedVS.html>`_ page.
 
+* The Loop Vectorizer that was previously enabled for -O3 is now enabled for
+  -Os and -O2.
+
+* The new SLP Vectorizer is now enabled by default.
+
 * ... next change ...
 
 .. NOTE