Better example for Middleware in README

[skip ci]

README.md

@@ -354,37 +354,56 @@ Get current user
     User user()
 
 
+### Authorization
+
+Check if a user has a specific role or superseding role
+
+    boolean is(string $role)
+
+```php
+if (!$auth->is('admin')) {
+    http_response_code(403);
+    echo "You're not allowed to see this page";
+    exit();
+}
+```
+
 ### Access control (middleware)
 
+Check if a user has a specific role or superseding role
+
+    Jasny\Authz\Middleware asMiddleware(callback $getRequiredRole)
+
 You can apply access control manually using the `is()` method. Alteratively, if you're using a PSR-7 compatible router
 with middleware support (like [Jasny Router](https://github.com/jasny/router)]).
 
+The `$getRequiredRole` callback should return a boolean, string or array of string.
+
+Returning true means a the request will only be handled if a user is logged in.
+
 ```php
 $auth = new Auth(); // Implements the Jasny\Authz interface
 
-$roure->add($auth->asMiddleware(function(ServerRequest $request) {
-    $route = $request->getAttribute('route');
-    return isset($route->auth) ? $route->auth : null;
+$router->add($auth->asMiddleware(function(ServerRequest $request) {
+    return strpos($request->getUri()->getPath(), '/account/') === 0; // `/account/` is only available if logged in
 }));
 ```
 
-### Authorization
-
-Check if a user has a specific role or superseding role
-
-    boolean is(string $role)
+If the `Auth` class implements authorization (`Authz`) and the callback returns a string, the middleware will check if
+the user is authorized for that role. If an array of string is returned, the user should be authorized for at least one
+of the roles.
 
 ```php
-if (!$auth->is('admin')) {
-    http_response_code(403);
-    echo "You're not allowed to see this page";
-    exit();
-}
+$auth = new Auth(); // Implements the Jasny\Authz interface
+
+$router->add($auth->asMiddleware(function(ServerRequest $request) {
+    $route = $request->getAttribute('route');
+    return isset($route->auth) ? $route->auth : null;
+}));
 ```
 
 ### Confirmation
 
-
 #### Signup confirmation
 
 Get a verification token. Use it in an url and set that url in an e-mail to the user.