README example code is outdated

[tv42]

"How it Works" example has

#[macro_use]
extern crate derive_builder;

which is old style Rust and not very recognizable/usable to newbies (https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html).

Below it, "Get Started" says "Add use derive_builder::Builder;", which is the new style.

[TedDriggs]

This relates to #231; I think we want to move all the tests and examples to the new style.