docs: replace gitter links with discord (#347)

README.md

@@ -6,7 +6,7 @@
 [![Javadocs](https://www.javadoc.io/badge/org.casbin/jcasbin.svg)](https://www.javadoc.io/doc/org.casbin/jcasbin)
 [![Maven Central](https://img.shields.io/maven-central/v/org.casbin/jcasbin.svg)](https://mvnrepository.com/artifact/org.casbin/jcasbin/latest)
 [![Release](https://img.shields.io/github/release/casbin/jcasbin.svg)](https://github.com/casbin/jcasbin/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)
 
 💖 [**Looking for an open-source identity and access management solution like Okta, Auth0, Keycloak ? Learn more about: Casdoor**](https://casdoor.org/)