Add support for compute network routing mode (hashicorp#838)

* Add ability to specify compute network routing mode
* Update docs

Author: mrparkers

docs/r/compute_network.html.markdown

@@ -40,6 +40,11 @@ The following arguments are supported:
   specified range. This API is deprecated. If set, `auto_create_subnetworks` must be
   explicitly set to false.
 
+* `routing_mode` - (Optional) Sets the network-wide routing mode for Cloud Routers
+  to use. Accepted values are `"GLOBAL"` or `"REGIONAL"`. Defaults to `"REGIONAL"`.
+  Refer to the [Cloud Router documentation](https://cloud.google.com/router/docs/concepts/overview#dynamic-routing-mode)
+  for more details.
+
 * `description` - (Optional) A brief description of this resource.
 
 * `project` - (Optional) The project in which the resource belongs. If it