Documented the security:check command

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4051
Do you really think is necessary? The installation chapter is for people that know nothing about Symfony. They're going to use the installer and they're going to install the standard edition, so everything works out of the box. My feel is that this note is only for advanced users doing custom installations.