Update documentation to specify Docker environment config required

[haxxordan]

On Windows at least, the docker-machine environment configuration is required for forward2docker to work properly, but nothing in the docs specify this and no error information indicating this is output.

Without this, at least on a Windows host, the VirtualBox VM's IP and port won't be detected properly.

From a terminal window, the output of the following command needs to be run first: docker-machine env --shell=cmd