Improve docs for Dummy derive

[xd009642]

So looking at the code I can see there's a lot more functionality than shown in the docs https://docs.rs/fake/latest/fake/derive.Dummy.html and I'm trying to figure out different ways to do the following:

• Enums and variants
• Custom types

So things like:

#[derive(Dummy)] 
enum Foo {
   #[dummy(faker = "0..50")] 
   Bar(i32)
}

I'm willing to help out with the docs via PRs just need some time to puzzle out how to do things