Update docs/configuring-playbook-prometheus-nginxlog.md: add the common section "Extending the configuration"

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-prometheus-nginxlog.md

@@ -20,6 +20,14 @@ Add the following configuration to your `inventory/host_vars/matrix.example.com/
 matrix_prometheus_nginxlog_exporter_enabled: true
 ```
 
+### Extending the configuration
+
+There are some additional things you may wish to configure about the component.
+
+Take a look at:
+
+- `roles/custom/matrix-prometheus-nginxlog-exporter/defaults/main.yml` for some variables that you can customize via your `vars.yml` file
+
 ## Installing
 
 After configuring the playbook, run it with [playbook tags](playbook-tags.md) as below: