add entry to the provider-contract

Author: mkumatag

docs/book/src/clusterctl/provider-contract.md

@@ -190,6 +190,7 @@ providers.
 |CACPN         | cluster.x-k8s.io/provider=control-plane-nested        |
 |CAPA          | cluster.x-k8s.io/provider=infrastructure-aws           |
 |CAPB          | cluster.x-k8s.io/provider=infrastructure-byoh          |
+|CAPIBM        | cluster.x-k8s.io/provider=infrastructure-ibmcloud      |
 |CAPV          | cluster.x-k8s.io/provider=infrastructure-vsphere       |
 |CAPD          | cluster.x-k8s.io/provider=infrastructure-docker        |
 |CAPM3         | cluster.x-k8s.io/provider=infrastructure-metal3        |