Replace Gitter with Zulip

Author: SylvainCorlay

README.md

@@ -6,7 +6,7 @@
 [![Documentation](http://readthedocs.org/projects/xtensor/badge/?version=latest)](https://xtensor.readthedocs.io/en/latest/?badge=latest)
 [![Doxygen -> gh-pages](https://github.com/xtensor-stack/xtensor/workflows/gh-pages/badge.svg)](https://xtensor-stack.github.io/xtensor)
 [![Binder](https://mybinder.org/badge.svg)](https://mybinder.org/v2/gh/xtensor-stack/xtensor/stable?filepath=notebooks%2Fxtensor.ipynb)
-[![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)
 
 Multi-dimensional arrays with broadcasting and lazy computing.