update logger doc string

src/middleware/logger.rs

@@ -13,6 +13,9 @@ use httpresponse::HttpResponse;
 use middleware::{Middleware, Started, Finished};
 
 /// `Middleware` for logging request and response info to the terminal.
+/// `Logger` middleware uses standard log crate to log information. You should
+/// enable logger for *actix_web* package to see access log.
+/// ([env_logger](https://docs.rs/env_logger/*/env_logger/) or similar)
 ///
 /// ## Usage
 ///
@@ -24,10 +27,14 @@ use middleware::{Middleware, Started, Finished};
 /// ```
 /// ```rust
 /// # extern crate actix_web;
+/// extern crate env_logger;
 /// use actix_web::Application;
 /// use actix_web::middleware::Logger;
 ///
 /// fn main() {
+///     std::env::set_var("RUST_LOG", "actix_web=info");
+///     env_logger::init();
+///
 ///     let app = Application::new()
 ///         .middleware(Logger::default())
 ///         .middleware(Logger::new("%a %{User-Agent}i"))