Remove gitter link and add link to Zulip channel.

Author: SylvainCorlay

README.md

@@ -2,7 +2,7 @@
 
 [![GithubActions](https://github.com/jupyter-xeus/xeus-zmq/actions/workflows/main.yml/badge.svg)](https://github.com/jupyter-xeus/xeus-zmq/actions/workflows/main.yml)
 [![Documentation Status](https://readthedocs.org/projects/xeus-zmq/badge/?version=latest)](https://xeus-zmq.readthedocs.io/en/latest/?badge=latest)
-[![Join the Gitter Chat](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/QuantStack/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Zulip](https://img.shields.io/badge/social_chat-zulip-blue.svg)](https://jupyter.zulipchat.com/#narrow/channel/539737-Jupyter-Kernels)
 
 ZeroMQ-based middleware for xeus