Add documentation of the 'required' field tag to the package comment.

Hello,

Been enjoying using your library, I've found it quite useful.

I just noticed that there's actually a "required" field tag option; not sure why it's used slightly inconsistantly with `OptionalArgument`, but I figured it could be documented.

Happy to go through and make these consistent if you like.. I suppose they should just resolve to the same option, with one being the inverse of the other?