docs(examples): print that examples are using only 1 thread

examples/hello.rs

@@ -32,6 +32,6 @@ fn main() {
     pretty_env_logger::init().unwrap();
     let addr = "127.0.0.1:3000".parse().unwrap();
     let server = Http::new().bind(&addr, || Ok(Hello)).unwrap();
-    println!("Listening on http://{}", server.local_addr().unwrap());
+    println!("Listening on http://{} with 1 thread.", server.local_addr().unwrap());
     server.run().unwrap();
 }