docs: replace gitter links with discord (#159)

README.md

@@ -4,7 +4,7 @@
 [![NPM download][download-image]][download-url]
 [![ci](https://github.com/node-casbin/nest-authz/actions/workflows/ci.yml/badge.svg)](https://github.com/node-casbin/nest-authz/actions/workflows/ci.yml)
 [![Coverage Status](https://coveralls.io/repos/github/node-casbin/nest-authz/badge.svg?branch=master)](https://coveralls.io/github/node-casbin/nest-authz?branch=master)
-[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/casbin/lobby)
+[![Discord](https://img.shields.io/discord/1022748306096537660?logo=discord&label=discord&color=5865F2)](https://discord.gg/S5UjpzGZjN)
 
 [npm-image]: https://img.shields.io/npm/v/nest-authz.svg?style=flat-square
 [npm-url]: https://npmjs.org/package/nest-authz