Update newtype_derive docs to use macro_attr!

[Rufflewind]

I was a bit confused why the crate used a deprecated dependency by the same author! Had to look in the tests to see what the new syntax is like.

[alberdingk-thijm]

I'd second this for enum_derive. The crate docs for enum_derive have:

custom_derive! {
    #[derive(Debug, PartialEq, Eq,
        IterVariants(CandyVariants), IterVariantNames(CandyVariantNames))]
    pub enum Candy { Musk, FruitRock, BoPeeps, LemonSherbert }
}

whereas this works now as

macro_attr! {
    #[derive(Debug, PartialEq, Eq,
        IterVariants!(CandyVariants), IterVariantNames!(CandyVariantNames))]
    pub enum Candy { Musk, FruitRock, BoPeeps, LemonSherbert }
}

[bluss]

I think newtype_derive just needs to be published, it looks up to date in git.