docs: replace gitter links with discord (#59)

Author: Selflocking

README.md

@@ -5,7 +5,7 @@ TypeORM Adapter
 [![codebeat badge](https://codebeat.co/badges/7b938f17-ac89-4ee9-b3cc-787b5e94720d)](https://codebeat.co/projects/github-com-node-casbin-typeorm-adapter-master)
 [![CI](https://github.com/node-casbin/typeorm-adapter/actions/workflows/ci.yml/badge.svg)](https://github.com/node-casbin/typeorm-adapter/actions/workflows/ci.yml)
 [![Coverage Status](https://coveralls.io/repos/github/node-casbin/typeorm-adapter/badge.svg?branch=master)](https://coveralls.io/github/node-casbin/typeorm-adapter?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/typeorm-adapter.svg?style=flat-square
 [npm-url]: https://npmjs.org/package/typeorm-adapter