Docs reminder to unset environmental variables

[mattidupre]

A lot of people will be trying this having already installed the Docker suite. Might want to make note of the need to unset environmental variables in order for docker and docker-compose to work.

unset DOCKER_HOST
unset DOCKER_CERT_PATH
unset DOCKER_TLS_VERIFY
unset DOCKER_MACHINE_NAME

Then again, I'm new to Docker so maybe it's obvious. Either way, great project! Loving it.

[nlf]

good idea! feel free to submit a pull request. otherwise i'll get to it when i have a few minutes