docs: add Slack invitation link badge (#2617)

diff --git a/README.md b/README.md
index baf3622..607775c 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,7 @@
 [![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/)
+[![Slack](https://badgen.net/badge/Slack/Join%20Apache%20APISIX?icon=slack)](https://apisix.apache.org/slack)
 
 <p align="center">
   <a href="https://apisix.apache.org/">Website</a> •