README: change chat badge from gitter to zulip

README.md

@@ -2,7 +2,7 @@
 ======
 
 [![](https://github.com/scifio/scifio/actions/workflows/build-main.yml/badge.svg)](https://github.com/scifio/scifio/actions/workflows/build-main.yml)
-[![Join the chat at https://gitter.im/scifio/scifio](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/scifio/scifio?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![developer chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://imagesc.zulipchat.com/#narrow/stream/327239-SCIFIO)
 
 SCIFIO (SCientific Image Format Input & Output) is an extensible Java framework
 for reading and writing images, particularly N-dimensional scientific images.