doc(Tutorial): Add explanation for app config (#4137)

docs/index.rst

@@ -218,3 +218,4 @@ Table Of Contents
    misc
    external
    contributing
+   tutorial/index

docs/tutorial/index.rst

@@ -0,0 +1,84 @@
+Tutorial
+========
+
+.. contents::
+   :local:
+
+
+Purpose of Tutorial
+-------------------
+
+
+`@Arfey <https://github.com/Arfey>`_ 
+`wrote <https://github.com/aio-libs/aiohttp/issues/4137#issuecomment-538248544>`_ :
+
+..
+
+    Tutorial is story about show how may looks project on aiohttp and how u need 
+    to design it. The main documentation is about how work and why.
+
+
+
+Why does Aiohttp choose sharing settings?
+-----------------------------------------
+
+There are `discussion <https://github.com/aio-libs/aiohttp/issues/2689>`_ 
+(and `this <https://github.com/aio-libs/aiohttp/issues/2412>`_ ) on 
+configurations for nested applictation (subapplication). Subapplications can be 
+separated entities, having their own configurations, for e.g. database or 
+cache purposes.
+
+Web developers want the main applictation to control the subapplications, 
+this requires a mechanism for the app to access the context.
+
+Centralization of configurations has the benefits listed as the following:
+
+1. **Easy management**
+
+In the case of subapplications exist, communication between main app and 
+subapplications is common. Aiohttp choose to set the configurations in 
+main app's context. Subapplications hence can access the settings at the 
+stage of initialization.
+
+This mechanism is similar to the implementation of Flask offering a 'g' variable.
+
+More details can be found `here <https://github.com/aio-libs/aiohttp/issues/2689>`_ .
+
+2. **Keep it simple**
+
+Keeping a list of db connection instances at runtime can achieve it. But it 
+comes up ordering problems, like managing the lifecycle of things like a 
+database connection, scheduler, etc.
+
+Conclusion
+^^^^^^^^^^
+
+Making use of ``request.config_dict`` 
+(`#2949 <https://github.com/aio-libs/aiohttp/pull/2949>`_ feature) is the way 
+out.
+
+An typical way to setup an app:
+
+.. code-block:: python
+
+    async def init_pg(app):
+        conf = app['config']['postgres']
+        engine = await aiopg.sa.create_engine(
+            database=conf['database'],
+            user=conf['user'],
+            password=conf['password'],
+            host=conf['host'],
+            port=conf['port'],
+            minsize=conf['minsize'],
+            maxsize=conf['maxsize'],
+        )
+        app['db'] = engine
+
+Further discussion
+^^^^^^^^^^^^^^^^^^
+
+`cleanup_ctx` is the mechanism to handle a subapplication starting up and 
+cleaning up. The discussion is ongoing ` (issue 
+`#3876 <https://github.com/aio-libs/aiohttp/issues/3876>`_). 
+`@mrasband <https://github.com/mrasband>`_ suggested to push subapplication 
+configurations to one of the key in the ``app`` mapping.