add note to docs about overlay + btrfs unsupported

Closes #9557. Ping @bjaglin do you think this is sufficient? :)

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Whoops sorry, will update
I feel like such a hypocrite feel free to smack me for the not reading the contributing guide