doc: op-build v1.21.2 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 1912751..b06ec9d 100644
--- a/doc/release-notes/index.rst
+++ b/doc/release-notes/index.rst
@@ -10,3 +10,5 @@
    v1.21-rc2
    v1.21
    v1.21.1
+   v1.21.2
+