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
I was pondering that too .. then again .. I feared that mentioning initializers or sonata here would be confusing