title: Adding a third party event subscriber
body: <p>I am relatively new to symfony, working on a project using PostgreSQL with postgis. Therefore I would like to make doctrine supporting postgis and stumbled upon this extension:
  <a href="https://github.com/jsor/doctrine-postgis" rel="nofollow">https://github.com/jsor/doctrine-postgis</a></p>
  
  <p>In the setup part are just two lines of code and a link referencing to the symfony documentation given.</p>
  
  <p>So what I tried was adding the ORMSchemaEventSubscriber in my services.yml file:</p>
  
  <pre><code>parameters:
  #    parameter_name: value
  
  services:
      my.subscriber:
          class: Jsor\Doctrine\PostGIS\Event\ORMSchemaEventSubscriber
          arguments: []
          tags:
              - { name: kernel.event_subscriber }
  </code></pre>
  
  <p>This approach throws an exception stating the ORMSchemaEventSubscriber class is no subclass of Symfony\Component\EventDispatcher\EventSubscriberInterface.</p>
  
  <p>That is where I am stuck and not knowing what to do or where to put the given lines of code.</p>