Slightly fuller usage message

You may think this is overkill, but I find the `--help` message and the error given if there's no `$VERSION` found to be too brief.

This is an attempt to add a slightly fuller error and `--help` message without too much overhead. (Your `README` gives good examples of how to use `rbfu`. This patch simply pulls those into `rbfu` itself, so the user doesn't need to refer to the `README` if they're confused at the command line.)