Update README.md

Saw some errors in my install that devicemapper was deprecated and that overlay2 is the recommended default:
https://docs.docker.com/engine/storage/drivers/select-storage-driver/

README.md

@@ -116,7 +116,7 @@ A list of system users to be added to the `docker` group (so they can use Docker
 
 ```yaml
 docker_daemon_options:
- storage-driver: "devicemapper"
+ storage-driver: "overlay2"
  log-opts:
  max-size: "100m"
 ```