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 <[email protected]>

docs/yml.md

@@ -75,6 +75,8 @@ expose:
 ### volumes
 
 Mount paths as volumes, optionally specifying a path on the host machine (`HOST:CONTAINER`).
+Note: Mapping local volumes is currently unsuported on boot2docker. We
+recommend you use docker-osx if want to map local volumes.
 
 ```
 volumes: