docs: replace gitter links with discord (casbin#218)

README.md

@@ -12,7 +12,7 @@ Gorm Adapter
 [![Coverage Status](https://coveralls.io/repos/github/casbin/gorm-adapter/badge.svg?branch=master)](https://coveralls.io/github/casbin/gorm-adapter?branch=master)
 [![Godoc](https://godoc.org/github.com/casbin/gorm-adapter?status.svg)](https://godoc.org/github.com/casbin/gorm-adapter)
 [![Release](https://img.shields.io/github/release/casbin/gorm-adapter.svg)](/~https://github.com/casbin/gorm-adapter/releases/latest)
-[![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)
 [![Sourcegraph](https://sourcegraph.com/github.com/casbin/gorm-adapter/-/badge.svg)](https://sourcegraph.com/github.com/casbin/gorm-adapter?badge)
 
 Gorm Adapter is the [Gorm](https://gorm.io/gorm) adapter for [Casbin](/~https://github.com/casbin/casbin). With this library, Casbin can load policy from Gorm supported database or save policy to it.