Update docs/configuring-playbook-bridge-matrix-bridge-sms.md: add the common section "Extending the configuration"

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

docs/configuring-playbook-bridge-matrix-bridge-sms.md

@@ -33,6 +33,14 @@ matrix_sms_bridge_provider_android_truststore_password: 123
 
 ```
 
+### Extending the configuration
+
+There are some additional things you may wish to configure about the bridge.
+
+Take a look at:
+
+- `roles/custom/matrix-bridge-sms/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_sms_bridge_configuration_extension_yaml` variable
+
 ## Installing
 
 After configuring the playbook, run it with [playbook tags](playbook-tags.md) as below: