`--help` should print usage info for shebang scripts

[lilyball]

The --help right now just lists all the flags. It should also include info on how to write shebang scripts, both the cargo manifest version and the cargo-deps: version. This way I don't need to go to the website to remember how to write these, especially as there's no manpage.

[lilyball]

I keep running into this and keep having to consult the website anew to find the // cargo-deps: syntax, as I use this infrequently enough that I don't actually remember how to do it.

[memark]

This would be great!

Do note that specifying cargo deps and using a shebang (#!/usr/bin/env rust-script) are not the same thing.