Add db initialization

Large-app-how-to.md

@@ -386,9 +386,16 @@ Here is the `/app/__init__.py`
     #app.register_blueprint(commentsModule)
     #app.register_blueprint(postsModule)
 
-now you can go in your root, activate your virtual env and run `python run.py` ... something like:
+Let's activate your virtual env and initialize your database:
 
     brice@Brice:~/Projects/dev$ . env/bin/activate
+    (env)brice@Brice:~/Projects/dev1$ python shell.py 
+    >>> from app import db
+    >>> db.create_all()
+    >>> exit()
+
+now you can go in your root and run `python run.py` ... should give you something like:
+
     (env)brice@Brice:~/Projects/dev$ python run.py 
      * Running on http://127.0.0.1:5000/
      * Restarting with reloader