doc: add v1.21 release notes

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
diff --git a/doc/release-notes/index.rst b/doc/release-notes/index.rst
index 605350d..c1ba934 100644
--- a/doc/release-notes/index.rst
+++ b/doc/release-notes/index.rst
@@ -8,3 +8,4 @@
 
    v1.21-rc1
    v1.21-rc2
+   v1.21