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 change this to:

````xml
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services">

    <services>
        <service id="acme_exception_subscriber"
            class="Acme\DemoBundle\EventSubscriber\AcmeExceptionSubscriber">

            <tag name="kernel.event_subscriber"/>

        </service>
    </services>
</container>
````