initial commit of a cookbook entry for how to setup REST read support

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | https://github.com/symfony-cmf/symfony-cmf-docs/issues/354

I am not sure where to best note the fact that the response structure of ContentBundle will change in 1.1, see https://github.com/symfony-cmf/ContentBundle/pull/91