docs(readme): add how to start the server

[hongbo-miao]

Just add how to start the server to be more user friendly. 😃

I originally start the server by

python app.py

it immediately quit, and I thought it is broken.

Took me a while to find the solution at https://github.com/pallets/flask#a-simple-example

flask run

[miguelgrinberg]

Sorry, but this is very misleading. There are many ways to structure a Flask app (not always with a app.py file), and there are many ways to start the Flask application (not always with flask run). This is not information that needs to be shared here, the user can learn about Flask in the Flask documentation.

[hongbo-miao]

Sounds good, it would be great to add that information to be more new user friendly! Thanks!