Add an example for private DNS Managed Zones (#2735)

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/cc @drebes

website/docs/r/dns_managed_zone.html.markdown

@@ -55,6 +55,50 @@ resource "random_id" "rnd" {
   byte_length = 4
 }
 ```
+## Example Usage - Dns Managed Zone Private
+
+
+```hcl
+resource "google_dns_managed_zone" "private-zone" {
+  name = "private-zone"
+  dns_name = "private.example.com."
+  description = "Example private DNS zone"
+  labels = {
+    foo = "bar"
+  }
+
+  visibility = "private"
+
+  private_visibility_config {
+    networks {
+      network_url =  "${google_compute_network.network-1.self_link}"
+    }
+    networks {
+      network_url =  "${google_compute_network.network-2.self_link}"
+    }
+  }
+
+  forwarding_config {
+    target_name_servers {
+      ipv4_address = "172.16.1.10"
+    }
+    target_name_servers {
+      ipv4_address = "172.16.1.20"
+    }
+  }
+
+}
+
+resource "google_compute_network" "network-1" {
+  name = "network-1"
+  auto_create_subnetworks = false
+}
+
+resource "google_compute_network" "network-2" {
+  name = "network-2"
+  auto_create_subnetworks = false
+}
+```
 
 ## Argument Reference