as the initializer is only run when sonata is present, ensure users create /cms/routes themselves

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | https://github.com/symfony-cmf/standard-edition/issues/23
should we add a comment here that this is not needed when using sonata admin as there will be the initializer? otherwise people will just copy this line into their project and never understand they might not need it.