add ngnix-status to config

add doc

docs/configuring-playbook-ngnix.md

@@ -0,0 +1,13 @@
+# Configure Ngnix (optional, advanced)
+
+By default, this playbook installs its own nginx webserver (in a Docker container) which listens on ports 80 and 443.
+If that's alright, you can skip this.
+
+
+## Using Ngnix status
+
+This will serve a statuspage to the hosting machine only. Useful for monitoring software like [longview](https://www.linode.com/docs/platform/longview/longview-app-for-nginx/)
+
+```yaml
+matrix_nginx_proxy_nginx_status_enabled: true
+```