Add mention of positional arguments.

[rpgoldman]

The existing documentation used the term "free arguments," but programmers familiar with other argument processing frameworks (e.g., Python's argparse) may be looking for the term "positional arguments," which is commonly used. This adds that, so a quick textual search of the README will show that command-arguments is the way to get positional arguments.

Addresses #21

[dnaeon]

Thanks, @rpgoldman !

[rpgoldman]

A pleasure to contribute!