v2.3.1 release notes

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
(cherry picked from commit 75d56215c09f6c489ee0eff03add2439f918b987)
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
diff --git a/doc/release-notes/index.rst b/doc/release-notes/index.rst
index dc1fe70..1f9feb5 100644
--- a/doc/release-notes/index.rst
+++ b/doc/release-notes/index.rst
@@ -76,3 +76,4 @@
    v2.3-rc1 
    v2.3-rc2
    v2.3
+   v2.3.1