add example of how to set up with init_app method (#302)

docs/index.rst

@@ -133,7 +133,23 @@ The following example initializes the extension with the standard Flask command-
     from flask_migrate import Migrate
     migrate = Migrate(app, db)
 
-The two arguments to ``Migrate`` are the application instance and the Flask-SQLAlchemy database instance. The ``Migrate`` constructor also takes additional keyword arguments, which are passed to Alembic's ``EnvironmentContext.configure()`` method. As is standard for all Flask extensions, Flask-Migrate can be initialized using the ``init_app`` method as well.
+The two arguments to ``Migrate`` are the application instance and the Flask-SQLAlchemy database instance. The ``Migrate`` constructor also takes additional keyword arguments, which are passed to Alembic's ``EnvironmentContext.configure()`` method. As is standard for all Flask extensions, Flask-Migrate can be initialized using the ``init_app`` method as well::
+
+    from flask_sqlalchemy import SQLAlchemy
+    from flask_migrate import Migrate
+
+    db = SQLAlchemy()
+    migrate = Migrate()
+
+    def create_app():
+         """Application-factory pattern"""
+         ...
+         ...
+         db.init_app(app)
+         migrate.init_app(app, db)
+         ...
+         ...
+         return app
 
 When using Flask-Script's command-line interface, the extension is initialized as follows::