Improve the static files example. (#317)

* Improve the static files example.

* Replace the broken websocket example with a simple static HTML page.
* Show how to serve a second directory which is located inside the main
  directory but has file listing enabled.
* Demonstrate that a hierarchy of static files can be served by placing the
  JS and CSS files in their respective subfolders.
* Document how to use the example in the README.

* Improve formatting.

static_index/src/main.rs

@@ -1,4 +1,4 @@
-use actix_files as fs;
+use actix_files::Files;
 use actix_web::{middleware, App, HttpServer};
 
 #[actix_rt::main]
@@ -8,12 +8,15 @@ async fn main() -> std::io::Result<()> {
 
     HttpServer::new(|| {
         App::new()
-            // enable logger
+            // Enable the logger.
             .wrap(middleware::Logger::default())
-            .service(
-                // static files
-                fs::Files::new("/", "./static/").index_file("index.html"),
-            )
+            // We allow the visitor to see an index of the images at `/images`.
+            .service(Files::new("/images", "static/images/").show_files_listing())
+            // Serve a tree of static files at the web root and specify the index file.
+            // Note that the root path should always be defined as the last item. The paths are
+            // resolved in the order they are defined. If this would be placed before the `/images`
+            // path then the service for the static images would never be reached.
+            .service(Files::new("/", "./static/root/").index_file("index.html"))
     })
     .bind("127.0.0.1:8080")?
     .run()