[symfony#10461] Add docs about custom environment

Author: wouterj

src/Symfony/Bundle/FrameworkBundle/Command/ServerRunCommand.php

@@ -66,6 +66,9 @@ protected function configure()
 router script using <info>--router</info> option:
 
   <info>%command.full_name% --router=app/config/router.php</info>
+  
+Specifing a router script is required when the used environment is not "dev" or
+"prod".
 
 See also: http://www.php.net/manual/en/features.commandline.webserver.php
 EOF