Fix and clean up CDC documentation

The CDC_users.md document was not linked to and was in the wrong place. It
should reside in the protocol directory since it relates to the CDC protocol.

Documentation/Protocols/CDC_users.md

@@ -0,0 +1,36 @@
+# Change Data Capture (CDC) users
+
+Change Data Capture (CDC) is a new MaxScale protocol that allows compatible
+clients to authenticate and register for Change Data Capture events.  The new
+protocol must be use in conjunction with AVRO router which currently converts
+MySQL binlog events into AVRO records.  Clients connect to CDC listener and
+authenticate using credentials provided in a format described in the [CDC Protocol documentation](CDC.md).
+
+**Note**: If no users are found in that file or if it doesn't exist, the only
+  available user will be the _service user_:
+
+```
+[avro-service]
+type=service
+router=avrorouter
+source=replication-service
+user=cdc_user
+password=cdc_password
+```
+
+## Creating new CDC users
+
+```
+bash$ cdc_users [-h] USER PASSWORD
+```
+
+The output of this command should be appended to the _cdcusers_ file at
+`/var/lib/maxscale/<service name>/`.
+
+```
+bash$ cdc_users user1 pass1 >> /var/lib/maxscale/avro-service/cdcusers
+```
+
+Users can be deleted by removing the related rows in 'cdcusers' file. For
+more details on the format of the _cdcusers_ file, read the [CDC Protocol documentation](CDC.md).
+