docs(managing_deis): add section for registrationMode

This commit adds a section to the managing_deis/production_deployment documentation
that explains how to disable or set registrations to adminOnly.

Author: Matt Dolian

docs/managing_deis/production_deployments.rst

@@ -53,6 +53,25 @@ Back up data
 
 Backing up data regularly is recommended. See :ref:`backing_up_data` for steps.
 
+Change Registration Mode
+------------------------
+
+Changing the registration process is highly recommended in production.  By default, registrations for a
+new cluster are open to anyone with the proper URL.  Once the admin user has registered on a new cluster,
+it is recommended that you either turn off registrations entirely or enable the adminOnly registration feature.
+
+Turn off registrations:
+
+.. code-block:: console
+
+   $ deisctl config platform set registrationMode=disable
+
+Turn on admin only registrations:
+
+.. code-block:: console
+
+   $ deisctl config platform set registrationMode=adminOnly
+
 Configure logging and monitoring
 --------------------------------