Documented the security:check command

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4051
we could add a hint, that the command is returning an error code if the security issue is found, so you could use it in a ci process. https://github.com/sensiolabs/security-checker/blob/master/SensioLabs/Security/Command/SecurityCheckerCommand.php#L98