Mention csi in persistent_volume_source (#1081)

Author: RolandG

website/docs/r/persistent_volume.html.markdown

@@ -139,6 +139,7 @@ The following arguments are supported:
 * `azure_file` - (Optional) Represents an Azure File Service mount on the host and bind mount to the pod.
 * `ceph_fs` - (Optional) Represents a Ceph FS mount on the host that shares a pod's lifetime
 * `cinder` - (Optional) Represents a cinder volume attached and mounted on kubelets host machine. For more info see http://releases.k8s.io/HEAD/examples/mysql-cinder-pd/README.md
+* `csi` - (Optional) CSI represents storage that is handled by an external CSI driver. For more info see [Kubernetes reference](https://kubernetes.io/docs/concepts/storage/volumes/#csi)
 * `fc` - (Optional) Represents a Fibre Channel resource that is attached to a kubelet's host machine and then exposed to the pod.
 * `flex_volume` - (Optional) Represents a generic volume resource that is provisioned/attached using an exec based plugin. This is an alpha feature and may change in future.
 * `flocker` - (Optional) Represents a Flocker volume attached to a kubelet's host machine and exposed to the pod for its usage. This depends on the Flocker control service being running