Added documentation for from_fn and wrap in middleware page (#451)

* Added documentation for from_fn and wrap in middleware page

* Update docs/middleware.md

---------

Co-authored-by: Rob Ede <robjtede@icloud.com>

examples/middleware/src/from_fn.rs

@@ -0,0 +1,24 @@
+#![allow(dead_code, unused_variables)]
+
+// <from-fn>
+use actix_web::{
+    body::MessageBody,
+    dev::{ServiceRequest, ServiceResponse},
+    middleware::{from_fn, Next},
+    App, Error,
+};
+
+async fn my_midleware(
+    req: ServiceRequest,
+    next: Next<impl MessageBody>,
+) -> Result<ServiceResponse<impl MessageBody>, Error> {
+    // pre-processing
+    next.call(req).await
+    // post-processing
+}
+
+#[actix_web::main]
+async fn main() {
+    let app = App::new().wrap(from_fn(my_midleware));
+}
+// </from-fn>

examples/middleware/src/main.rs

@@ -1,5 +1,6 @@
 pub mod default_headers;
 pub mod errorhandler;
+pub mod from_fn;
 pub mod logger;
 pub mod user_sessions;
 pub mod wrap_fn;