Add instructions for creating Stripe plans

I had trouble getting Stripe to work in development following the "[getting started](https://www.youtube.com/watch?v=6xXOfQFj98g&index=4&list=PLhlPwpqjsgvXK4n8FJBbj7KkvuOw8h3FO)" Youtube video.

It turns out you have to create the plans on your own Stripe account manually.

This PR updates the documentation to reflect this.