Add a release not about llvm-ar.

Thanks to Bill Wendling for the reminder.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187794 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 00c848f..7b143f6 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -67,6 +67,9 @@
 
 * The new SLP Vectorizer is now enabled by default.
 
+* llvm-ar now uses the new Object library and produces archives and
+  symbol tables in the gnu format.
+
 * ... next change ...
 
 .. NOTE