add the tip for SQLAlchemy db and model namespace

Large-app-how-to.md

@@ -424,6 +424,8 @@ Here is the `/app/__init__.py` :
     #app.register_blueprint(postsModule)
 ```
 
+Our SQLAlchemy db instance and models (User) are separated in two files, you need import both of them into namespace, which is done by `from app.users.views import mod as usersModule` implicitly. Otherwise the `db.create_all()` will do nothing.
+
 Let's activate your virtualenv and initialize your database:
 
     user@Machine:~/Projects/dev$ . env/bin/activate