Document arguments in README

[Ben Greenier]

There are several supported arguments that should get documented in the README. They are:

var control = flag.String("control", "unix:///var/run/docker.sock", "Docker control socket")
var prefix = flag.String("prefix", "", "Docker container prefix to limit observation to")
var interval = flag.Int64("interval", 5000, "Interval to poll at (in ms)")
var retries = flag.Int64("retries", 10, "Max retry count for failed docker commands")