improved app doc

doc/app.md

@@ -17,3 +17,27 @@ To start the app do the following:
 ```python
 HelloApp().serve()
 ```
+
+The App can be configured by defining its init method:
+
+```python
+class HelloApp(appier.App):
+
+    def __init__(self):
+        appier.App.__init__(
+            self,
+            name = "app_name"
+        )
+```
+
+The basic `App` should seldom be inherited from, instead opt for inheriting from  `APIApp`
+or `WebApp`, depending on whether you're building just an API or a complete Web App with
+an user interface. These will provide default behaviours that are more appropriate to each
+scenario. For example, by inheriting from `WebApp` instead, the following behaviours are
+done by default:
+
+* Provide a default handler for the index route, so that the developer can visit the app
+through the browser the moment he launches it (without writing any handlers yet).
+* Redirects the user to the login page when acess is denied to a particular resource.
+* Provides a default HTML error page.
+* Defaults 'service' to False.