include instructions in the README for people to add volume

[jmsunseri]

Include instructions for people to add a volume and how to mount it so that they can edit outside of the docker container.

[binatrix]

docker run --name=RebbleSDK -v $(pwd)/pebble:/root/pebble -it rebble/pebble-sdk

where "pebble" is the shared directory between the container and the host. It may be necessary to take ownership of the directory on the host (sudo chown -R myhostuser:root pebble/) to allow editing.