docs(README.md): add beta badge (#894)

Author: chimurai

README.md

@@ -4,6 +4,7 @@
 [![Coveralls](https://img.shields.io/coveralls/chimurai/http-proxy-middleware.svg?style=flat-square&logo=coveralls)](https://coveralls.io/r/chimurai/http-proxy-middleware)
 [![Snyk Vulnerabilities for GitHub Repo](https://img.shields.io/snyk/vulnerabilities/github/chimurai/http-proxy-middleware?logo=snyk&style=flat-square)](https://security.snyk.io/package/npm/http-proxy-middleware)
 [![npm](https://img.shields.io/npm/v/http-proxy-middleware?color=%23CC3534&style=flat-square&logo=npm)](https://www.npmjs.com/package/http-proxy-middleware)
+[![npm (tag)](https://img.shields.io/npm/v/http-proxy-middleware/beta?color=CC3534&logo=npm&style=flat-square)](https://github.com/chimurai/http-proxy-middleware/discussions/768)
 
 Node.js proxying made simple. Configure proxy middleware with ease for [connect](https://github.com/senchalabs/connect), [express](https://github.com/expressjs/express), [next.js](https://github.com/vercel/next.js) and [many more](#compatible-servers).