Add info on config-less docker to the generic image readme.

docker/generic/README.md

@@ -9,6 +9,19 @@ Three docker volumes have been created in the image to be used for configuration
 /mosquitto/log
 ```
 
+## Running without a configuration file
+Mosquitto 2.0 requires you to configure listeners and authentication before it
+will allow connections from anything other than the loopback interface. In the
+context of a container, this means you would normally need to provide a
+configuration file with your settings.
+
+If you wish to run mosquitto without any authentication, and without setting
+any other configuration options, you can do so by using a configuration
+provided in the container for this purpose:
+```
+docker run -it -p 1883:1883 eclipse-mosquitto:<version> mosquitto -c /mosquitto-no-auth.conf
+```
+
 ## Configuration
 When creating a container from the image, the default configuration values are used.
 To use a custom configuration file, mount a **local** configuration file to `/mosquitto/config/mosquitto.conf`