Document in README how to install and use

The tool itself has a pretty nice `--help` output, once you get to running it at all.

When coming upon the repo here on GitHub, you don't see that and instead you see the README. It'd be really helpful for that to (a) say how to install it (it's trivial, in fact -- copy the files into `/usr/local/bin/` or elsewhere on your PATH -- but that needs to be written down), and (b) maybe just have a copy of the `--help` output, so you can see what you'll get.