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
maybe have a comment here saying "or configure an initializer for this path", and a `.. note::` after the code explaining that for routing, the routing initializer is only activated if a base path for sonata admin is configured, and link to the initializer and sonata docs.

i think not explaining what we do has more potential for confusion than linking the corresponding sections. a "note" to me means a reader can ignore it if it does not make sense to him.