Replace gitter link with Zulip link

Author: SylvainCorlay

README.md

@@ -11,7 +11,7 @@
 [![GHA macos](https://github.com/xtensor-stack/xsimd/actions/workflows/macos.yml/badge.svg)](https://github.com/xtensor-stack/xsimd/actions/workflows/macos.yml)
 [![GHA windows](https://github.com/xtensor-stack/xsimd/actions/workflows/windows.yml/badge.svg)](https://github.com/xtensor-stack/xsimd/actions/workflows/windows.yml)
 [![Documentation Status](http://readthedocs.org/projects/xsimd/badge/?version=latest)](https://xsimd.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://xtensor.zulipchat.com/#narrow/channel/539553-Ask-anything)
 
 C++ wrappers for SIMD intrinsics