How to extend/override Rest EntityResource Plugin in Drupal 8
To get this working I needed to:


Install the Rest UI module. 
Go to /admin/config/services/rest and
enable my new resource.
Also at /admin/config/services/rest, click the edit button next to the new resource and configure it.
Go to /admin/people/permissions and configure the permissions for the new resource (Access GET, Access POST, etc.)


Couple of notes...  Both on the Rest UI config and permissions pages, it was difficult to distinguish between my new resource and the original.  In addition, when I tried to disable the original resource from the Rest UI config page I got the following error: 

Fatal error: Call to undefined function Drupal\restui\Controller\drupal_valid_token() in drupal-8.0.0-beta2/modules/contrib/restui/src/Controller/RestUIController.php on line 211


However, even though both resources are configured, the system appears to be using my resource and not the original.