Update docs/configuring-playbook-appservice-double-puppet.md: add the common section "Extending the configuration"

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

docs/configuring-playbook-appservice-double-puppet.md

@@ -14,6 +14,14 @@ To enable the Appservice Double Puppet service, add the following configuration
 matrix_appservice_double_puppet_enabled: true
 ```
 
+### Extending the configuration
+
+There are some additional things you may wish to configure about the service.
+
+Take a look at:
+
+- `roles/custom/matrix-appservice-double-puppet/defaults/main.yml` for some variables that you can customize via your `vars.yml` file. You can override settings (even those that don't have dedicated playbook variables) using the `matrix_appservice_double_puppet_registration_configuration_extension_yaml` variable
+
 ## Installing
 
 After configuring the playbook, run it with [playbook tags](playbook-tags.md) as below: