docs: replace gitter links with discord (#23)

Author: Selflocking

README.md

@@ -5,7 +5,7 @@
 [![Version](https://img.shields.io/pypi/v/casbin-postgresql-watcher.svg)](https://pypi.org/project/casbin-postgresql-watcher/)
 [![PyPI - Wheel](https://img.shields.io/pypi/wheel/casbin-postgresql-watcher.svg)](https://pypi.org/project/casbin-postgresql-watcher/)
 [![Download](https://img.shields.io/pypi/dm/casbin-postgresql-watcher.svg)](https://pypi.org/project/casbin-postgresql-watcher/)
-[![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)
 
 Casbin role watcher to be used for monitoring updates to casbin policies
 ## Installation