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

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

docs/configuring-playbook-client-hydrogen.md

@@ -37,6 +37,15 @@ After changing the domain, **you may need to adjust your DNS** records to point
 
 If you've decided to reuse the `matrix.` domain, you won't need to do any extra DNS configuration.
 
+### Extending the configuration
+
+There are some additional things you may wish to configure about the client.
+
+Take a look at:
+
+- `roles/custom/matrix-client-hydrogen/defaults/main.yml` for some variables that you can customize via your `vars.yml` file
+- `roles/custom/matrix-client-hydrogen/templates/config.json.j2` for the client's default configuration. You can override settings (even those that don't have dedicated playbook variables) using the `matrix_client_hydrogen_configuration_extension_json` variable
+
 ## Installing
 
 After configuring the playbook and potentially [adjusting your DNS records](#adjusting-dns-records), run the playbook with [playbook tags](playbook-tags.md) as below: