Update docs/configuring-playbook-email2matrix.md: add the common section "Extending the configuration"

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

docs/configuring-playbook-email2matrix.md

@@ -100,6 +100,14 @@ where:
 
 Refer to the official documentation [here](https://github.com/devture/email2matrix/blob/master/docs/configuration.md).
 
+### Extending the configuration
+
+There are some additional things you may wish to configure about the component.
+
+Take a look at:
+
+- `roles/custom/matrix-email2matrix/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: