Add the 'required' option to the documentation

[Yevhenii Reizner]

Currently, it doesn't mentioned anywhere.

And it will be nice to have a list of all possible options/attributes.

[Murarth]

Full documentation of the #[options(...)] attribute is available in the docs for gumdrop_derive.

I've added a link in the crate-level docs for gumdrop that points to the gumdrop_derive docs.