Add cookbook entry about url trailing slash redirect

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.2+
| Fixed tickets | none

Like @WouterJ said in [this symfony CMF discussion](https://github.com/symfony-cmf/symfony-cmf-docs/pull/323), it may be interesting to have this in the symfony cookbook.