Link to Rust design patterns

Author: dtolnay

src/external-links.md

@@ -7,6 +7,7 @@
 - [RFC 1574] - Doc conventions
 - [RFC 1687] - Crate-level documentation
 - [Elegant Library APIs in Rust](https://deterministic.space/elegant-apis-in-rust.html)
+- [Rust Design Patterns](https://github.com/rust-unofficial/patterns)
 
 [RFC 344]: https://github.com/rust-lang/rfcs/blob/master/text/0344-conventions-galore.md
 [RFC 430]: https://github.com/rust-lang/rfcs/blob/master/text/0430-finalizing-naming-conventions.md