Add subcommand examples to README

[bosr]

Hi, thanks for putting all this together! Additionally, that's a great use case for Property Wrappers.

Could you please add subcommand examples to the README?

Thanks!

PS: I would have liked to contribute this, but I find it hard to enter the codebase because of the "meta" nature of most names in this domain: argument, option, argumenttype, etc, and I lack time to do so.