Add reference to `derive_builder`

patterns/builder.md

@@ -98,6 +98,8 @@ as well as the `FooBuilder::new().a().b().finish()` style.
 
 [Description in the style guide](https://doc.rust-lang.org/1.0.0/style/ownership/builders.html)
 
+[derive_builder](https://crates.io/crates/derive_builder), a crate for automatically implementing this pattern while avoiding the boilerplate.
+
 [Constructor pattern](../idioms/ctor.md) for when construction is simpler.
 
 [Builder pattern (wikipedia)](https://en.wikipedia.org/wiki/Builder_pattern)