Update release notes for stable/stein

Change-Id: Ifc5a36743c98f32a9d841636119bc0d33621cef3

releasenotes/source/index.rst

@@ -6,6 +6,7 @@ Kayobe Release Notes
    :maxdepth: 1
 
    unreleased
+   stein
    rocky
    queens
    pike

releasenotes/source/stein.rst

+==================================
+Stein (6.x.y) Series Release Notes
+==================================
+
+.. release-notes::
+   :branch: stable/stein
+   :earliest-version: 6.0.0.0rc1
\ No newline at end of file