docs: add note to volume-plugins
docs: add note to volume-plugins

The /var/lib/docker directory is reserved for
docker, and should not be touched by plugins.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

e11137/cli,redpanda/cli,allencloud/cli,allencloud/cli,riyazdf/cli,e11137/cli,docker/cli,docker/cli,Zebrilee/cli,redpanda/cli,dnephin/cli,thaJeztah/cli,thaJeztah/cli,Zebrilee/cli,ashfall/cli,cpuguy83/cli,dnephin/cli,ashfall/cli,riyazdf/cli,cpuguy83/cli