Added "How to create an event subscriber"

I've added the information about how to create an event subscriber to the "How to create an Event listener" cookbook recipe. I think that there isn't a recipe in the cookbook about that and it can be useful.
could you please line wrap this:

````php
$container
    ->register(
        'acme_exception_subscriber',
        'Acme\DemoBundle\EventSubscriber\AcmeExceptionSubscriber'
    )
    ->addTag('kernel.event_subscriber')
;
````