Improve swagger spec.

[andrewtsw]

I've added support for XML comments in a swagger spec. It is just a sample for the CategoriesController. But the same approach can be used for all other controllers and entities. It is very useful to show additional information like comments and examples in a swagger spec.

But if you use attributes - you can add information about optional/required fields to the spec. Or maxlen for the strings. So I believe better to use attributes for this kind of validation, instead of FluentValidation.

@jasontaylordev what do you think about this approach?

Thanks,