Update codegen-backend.md

src/doc/unstable-book/src/compiler-flags/codegen-backend.md

@@ -12,6 +12,9 @@ backend. The library must be of crate type `dylib` and must contain a function
 named `__rustc_codegen_backend` with a signature of `fn() -> Box<dyn rustc_codegen_ssa::traits::CodegenBackend>`.
 
 ## Example
+See also the [`hotplug_codegen_backend`](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/hotplug_codegen_backend) test
+for a full example.
+
 ```rust,ignore
 use rustc_codegen_ssa::traits::CodegenBackend;