Add lists of values to module documentation

[jon-edward]

This adds reference to nargs='+' as a method of handling lists of arguments to the module docstring

[jon-edward]

This is a workaround for #63.

[mivade]

I think this is probably the right way to handle #69 since just specifying list[str] (or whatever) can be ambiguous from the perspective of the nargs kwarg.

Closes #69.

[mivade]

Thanks @jon-edward!