`help` magic option is undocumented?

[icefoxen]

As far as I can tell, if you put a help option in your options struct, gumdrop magically notices it and adds behavior to print out a help messages for you. Also as far as I can tell, nowhere in the docs or examples does it actually talk about this.

[Murarth]

I've submitted PR #37 to address this issue.
The documentation is added to the example in src/lib.rs. I hope the wording is clear and concise. Please let me know what you think.