add note to docs about overlay + btrfs
add note to docs about overlay + btrfs

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)

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