Updated documentation

[Warfields]

Which crate to use was not obvious since serde_with_macros appears as its own crate on docs.rs

[Warfields]

To prevent issues like #59

[jonasbb]

Hi @Warfields, thanks for the effort. Unfortunately, I don't think it is possible to do exactly what you were planning here. The serde_with crate cannot be used from within serde_with_macros, as otherwise this would create a circular dependency between the two crates.

I would prefer to use the #[doc(inline)] attribute on the proc_macro, but this doesn't work, since there is a rustc limitation right now (#49).
I will improve the documentation with adding some hints, that everything from serde_with_macros is re-exported in serde_with.