doc: add app.router documentation

_includes/api/en/5x/app-router.md

@@ -0,0 +1,19 @@
+<h3 id='app.router'>app.router</h3>
+
+The application's in-built instance of router. This is created lazily, on first access.
+
+```js
+var express = require('express')
+var app = express()
+var router = app.router
+
+router.get('/', function (req, res) {
+  res.send('hello world')
+})
+
+app.listen(3000)
+```
+
+You can add middleware and HTTP method routes to the `router` just like an application.
+
+For more information, see [Router](#router).

_includes/api/en/5x/app.md

@@ -38,6 +38,10 @@ The Express application object can be referred from the [request object](#req) a
   {% include api/en/5x/app-mountpath.md %}
 </section>
 
+<section markdown="1">
+  {% include api/en/5x/app-router.md %}
+</section>
+
 <h3 id='app.events'>Events</h3>
 
 <section markdown="1">

_includes/api/en/5x/menu.md

@@ -18,6 +18,8 @@
             </li>
             <li><a href="#app.mountpath">app.mountpath</a>
             </li>
+            <li><a href="#app.router">app.router</a>
+            </li>
             <li><em>Events</em>
             </li>
             <li><a href="#app.onmount">mount</a>