Switch from Gitter to Zulip (#619)

[skip ci]

Author: nakamura-to

README.md

@@ -13,8 +13,8 @@ Doma has various strengths:
 [![Build Status](https://github.com/domaframework/doma/workflows/Java%20CI%20with%20Gradle/badge.svg)](https://github.com/domaframework/doma/actions?query=workflow%3A%22Java+CI+with+Gradle%22)
 [![docs](https://readthedocs.org/projects/doma/badge/?version=latest)](https://doma.readthedocs.io/en/latest/)
 [![javadoc](https://javadoc.io/badge2/org.seasar.doma/doma-core/javadoc.svg)](https://javadoc.io/doc/org.seasar.doma/doma-core)
-[![Join the chat at https://gitter.im/domaframework/doma](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/domaframework/doma?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Google group : doma-user](https://img.shields.io/badge/Google%20Group-doma--user-orange.svg)](https://groups.google.com/g/doma-user)
+[![project chat](https://img.shields.io/badge/zulip-join_chat-green.svg)](https://domaframework.zulipchat.com)
 [![Twitter](https://img.shields.io/badge/[email protected]?style=flat)](https://twitter.com/domaframework)
 
 Examples