doc: using middleware from registry at ring-handler level

doc/ring/middleware_registry.md

@@ -52,6 +52,20 @@ Router creation fails fast if the registry doesn't contain the middleware:
 ;| :bonus | reitit.ring_test$wrap_bonus@59fddabb |
 ```
 
+Middleware defined in the registry can also be used on the `ring-handler` level:
+
+```clj
+(def app
+  (ring/ring-handler
+    (ring/router
+      ["/api"
+       ["/bonus" {:get (fn [{:keys [bonus]}]
+                         {:status 200, :body {:bonus bonus}})}]]
+      {::middleware/registry {:bonus wrap-bonus}})
+    nil
+    {:middleware [[:bonus 15]]}))
+```
+
 ## When to use the registry?
 
 Middleware as Keywords helps to keep the routes (all but handlers) as literal data (i.e. data that evaluates to itself), enabling the routes to be persisted in external formats like EDN-files and databases. Duct is a good example, where the [middleware can be referenced from EDN-files](https://github.com/duct-framework/duct/wiki/Configuration). It should be easy to make Duct configuration a Middleware Registry in `reitit-ring`.