Update the documentation of the kubernetes cluster

Signed-off-by: Alejandro JNM <alejandrojnm@gmail.com>

website/docs/r/kubernetes_cluster.html.markdown

@@ -86,6 +86,7 @@ In addition to the arguments listed above, the following additional attributes a
 * `ready` -If the Kubernetes cluster is ready.
 * `kubeconfig` - A representation of the Kubernetes cluster's kubeconfig in yaml format.
 * `api_endpoint` - The base URL of the API server on the Kubernetes master node.
+* `master_ip` - The Ip of the Kubernetes master node.
 * `dns_entry` - The unique dns entry for the cluster in this case point to the master.
 * `built_at` - The date where the Kubernetes cluster was build.
 * `created_at` - The date where the Kubernetes cluster was create.