Add bit about wrap_fn to middlewares page (#104)

examples/middleware/src/main.rs

@@ -2,6 +2,7 @@ pub mod default_headers;
 pub mod errorhandler;
 pub mod logger;
 pub mod user_sessions;
+pub mod wrap_fn;
 
 // <simple>
 use actix_service::{Service, Transform};

examples/middleware/src/wrap_fn.rs

@@ -0,0 +1,20 @@
+// <wrap-fn>
+use actix_service::Service;
+use actix_web::{web, App};
+use futures::future::Future;
+
+fn main() {
+    let app = App::new()
+        .wrap_fn(|req, srv| {
+            println!("Hi from start. You requested: {}", req.path());
+            srv.call(req).map(|res| {
+                println!("Hi from response");
+                res
+            })
+        })
+        .route(
+            "/index.html",
+            web::get().to(|| "Hello, middleware!"),
+        );
+}
+// </wrap-fn>