docs: add cypress badge (#1937)

Author: LiteSun

README.md

@@ -22,6 +22,7 @@
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/apache/apisix-dashboard/blob/master/LICENSE)
 [![Go Report Card](https://goreportcard.com/badge/github.com/apache/apisix-dashboard)](https://goreportcard.com/report/github.com/apache/apisix-dashboard)
 [![DockerHub](https://img.shields.io/docker/pulls/apache/apisix-dashboard.svg)](https://hub.docker.com/r/apache/apisix-dashboard)
+[![Cypress.io](https://img.shields.io/badge/tested%20with-Cypress-04C38E.svg)](https://www.cypress.io/)
 
 <p align="center">
   <a href="https://apisix.apache.org/">Website</a> •