Add docs for #577.

website/docs/r/container_cluster.html.markdown

@@ -14,7 +14,7 @@ and
 [API](https://cloud.google.com/container-engine/reference/rest/v1/projects.zones.clusters).
 
 !> **Warning:** Due to limitations of the API, all arguments except
-`node_version` are non-updateable. Changing any will cause recreation of the
+`node_version` and `min_master_version` are non-updateable. Changing any will cause recreation of the
 whole cluster!
 
 ~> **Note:** All arguments including the username and password will be stored in the raw state as plain-text.
@@ -98,6 +98,12 @@ resource "google_container_cluster" "primary" {
 
 * `node_pool` - (Optional) List of node pools associated with this cluster.
 
+* `min_master_version` - (Optional) The minimum version of the master. GKE
+  can auto-update the master to new versions, so this does not guarantee the
+  current master version--use the read-only `master_version` field to obtain that.
+  Defaults to the default version set by GKE which is not necessarily the latest
+  version.
+
 * `node_version` - (Optional) The Kubernetes version on the nodes. Also affects
     the initial master version on cluster creation. Updates affect nodes only.
     Defaults to the default version set by GKE which is not necessarily the latest
@@ -200,6 +206,10 @@ exported:
 * `instance_group_urls` - List of instance group URLs which have been assigned
     to the cluster
 
+* `master_version` - The current version of the master in the cluster, which may
+    be different than the `min_master_version` set in the config, if the master
+    has been updated by GKE.
+
 * `master_auth.client_certificate` - Base64 encoded public certificate
     used by clients to authenticate to the cluster endpoint.
 
@@ -225,4 +235,4 @@ Container clusters can be imported using the `zone`, and `name`, e.g.
 
 ```
 $ terraform import google_container_cluster.mycluster us-east1-a/my-cluster
-```
+```