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)
This should be a properly formatted note - https://github.com/docker/docker/blob/master/docs/sources/contributing/docs_style-guide.md#notes