docs: add chat badge to README (#8006)

Author: mtrezza

README.md

@@ -20,6 +20,7 @@
 [![License][license-svg]][license-link]
 [![Forum](https://img.shields.io/discourse/https/community.parseplatform.org/topics.svg)](https://community.parseplatform.org/c/parse-server)
 [![Twitter](https://img.shields.io/twitter/follow/ParsePlatform.svg?label=Follow&style=social)](https://twitter.com/intent/follow?screen_name=ParsePlatform)
+[![Chat](https://img.shields.io/badge/Chat-Join!-%23fff?style=social&logo=slack)](https://chat.parseplatform.org)
 
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