update doc to talk about microversion

Author: jichenjc

README.md

@@ -73,6 +73,8 @@ Each version of Cluster API for OpenStack will attempt to support two Kubernetes
 **NOTE:** As the versioning for this project is tied to the versioning of Cluster API, future modifications to this
 policy may be made to more closely aligned with other providers in the Cluster API ecosystem.
 
+**NOTE:** The minimum microversion of CAPI using nova is `2.53` now due to `server tags` support, see [code](https://github.com/kubernetes-sigs/cluster-api-provider-openstack/blob/c052e7e600f0e5ebddc839c08746bb636e79be87/pkg/cloud/services/compute/service.go#L38) for additional information.
+
 ------
 
 ## Development versions