Add section in docs for subnetwork_project

Author: danawillow

website/source/docs/providers/google/r/compute_instance.html.markdown

@@ -136,10 +136,13 @@ The `network_interface` block supports:
 * `network` - (Optional) The name or self_link of the network to attach this interface to.
     Either `network` or `subnetwork` must be provided.
 
-*  `subnetwork` - (Optional) the name of the subnetwork to attach this interface
+*  `subnetwork` - (Optional) The name of the subnetwork to attach this interface
     to. The subnetwork must exist in the same region this instance will be
     created in. Either `network` or `subnetwork` must be provided.
 
+*  `subnetwork_project` - (Optional) The project in which the subnetwork belongs.
+   If it is not provided, the provider project is used.
+
 * `address` - (Optional) The private IP address to assign to the instance. If
     empty, the address will be automatically assigned.