How can I add coupons?

When you have no active coupons in your database, Sjabloon hides the “have a coupon?” link (and the following form) from the checkout page as coupons are not feasible for every app. Coupons can be easily added with the following:

1. Add your coupons in your Stripe account
2. Run rake stripe:fetch:coupons from the command line
3. The coupon form will now show on your checkout form