Update upgrade README and add 3.7.x -> 3.9.x entry

playbooks/byo/openshift-cluster/upgrades/README.md

@@ -4,5 +4,6 @@ cluster. Additional notes for the associated upgrade playbooks are
 provided in their respective directories.
 
 # Upgrades available
+- [OpenShift Container Platform 3.7 to 3.9](v3_6/README.md) (works also to upgrade OpenShift Origin from 3.7.x to 3.9.x)
 - [OpenShift Container Platform 3.6 to 3.7](v3_7/README.md) (works also to upgrade OpenShift Origin from 3.6.x to 3.7.x)
 - [OpenShift Container Platform 3.5 to 3.6](v3_6/README.md) (works also to upgrade OpenShift Origin from 1.5.x to 3.6.x)