doc: Add link to daemon.json upstream doc

roles/docker/README.md

@@ -3,6 +3,8 @@ Docker
 
 Ensures docker package or system container is installed, and optionally raises timeout for systemd-udevd.service to 5 minutes.
 
+daemon.json items may be found at https://docs.docker.com/engine/reference/commandline/dockerd/#daemon-configuration-file
+
 Requirements
 ------------