message: adds link and updates README
diff: @@ -53,7 +53,7 @@ interface InputData {
  locale: string;
  /** Shop where the checkout/order is from. */
  shop: Shop;
  - /** JWT representing the input_data payload. */
  + /** JWT representing the input_data payload. For more information, refer to [JWT specification](/api/checkout-extensions/jwt-specification). */
  token: string;
  /** Post Purchase API version. */
  version: string;
  