Add warning for mapping local volumes

Mapping local volumes is not currently supported in boot2docker

https://github.com/boot2docker/boot2docker/issues/413
https://github.com/dotcloud/docker/issues/4023

Signed-off-by: Saul Shanabrook <s.shanabrook@gmail.com>
Merge pull request #288 from saulshanabrook/patch-1

Add warning for mapping local volumes