DEV: Deprecate the plugin (#260)

Author: cvx

README.md

@@ -1,5 +1,8 @@
 # Chat Integration Plugin
 
+> [!IMPORTANT]
+> This plugin has now been bundled into Discourse core. See: https://meta.discourse.org/t/bundling-more-popular-plugins-with-discourse-core/373574
+
 Integrate your chat system of choice with Discourse.
 
 For more information, please see: https://meta.discourse.org/t/chatroom-integration-plugin-discourse-chat-integration/66522