README addition

[cmertes]

I also had to add my user to the docker group before launching the container:

$ sudo usermod -aG docker $(whoami)

[kwmsmith]

@cmertes Thanks -- what was the error message? I'll add this to the README

[kwmsmith]

Also, what OS / platform are you on?

[cmertes]

I'm on Ubuntu 16.04. With the Docker daemon already running:

Cannot connect to the Docker daemon. Is the docker daemon running on this host?

To fix, added myself to the docker group and restarted the terminal session.

Thanks much!