Add service hook for Packagist #6

Hi there, some time ago I added ZendOpenId to Packagist (see here) as it wasn't on there. I added zendframework as a maintainer, so you should have access to it. I'm not sure how to remove myself as a maintainer though, but that's not your issue :)

However, Packagist recommends adding GitHub service hooks to update. Instructions, for your convenience (from here):

Enabling the Packagist service hook ensures that your package will always be updated instantly when you push to GitHub. To do so you can go to your GitHub repository, click the "Settings" button, then "Service Hooks". Pick "Packagist" in the list, and add your API token (see above), plus your Packagist username if it is not the same as on GitHub. Check the "Active" box and submit the form.

Thanks!