Add stress test report

docs/plugins/flash-sales/README.md

@@ -44,3 +44,7 @@ A flash-sales plugin for EShop.
 1. Use `/api/e-shop/plugins/flash-sales/flash-sale-plan/{planId}/pre-order` (POST) to pre-order. It will return an expiration time if your pre-order request succeeds. Don't forget to re-invoke this API to refresh your request before the expiration time.
 2. When the flash sale starts, use `/api/e-shop/plugins/flash-sales/flash-sale-plan/{planId}/order` (POST) to create your order. If you are fast enough, it will occupy the inventory and create an order for you in the background.
 3. If you are told that you have succeeded, continuous use `/api/e-shop/plugins/flash-sales/flash-sale-result/current/{planId}` (GET) to query the order creation result until it succeeds or fails.
+
+## Stress Test
+
+See the [stress test report](https://github.com/gdlcf88/eshop-flashsales-k6-stress-test/tree/main/EShop3.0Preview18-standalone).