Add slack badge to README

Author: kelson42

README.md

@@ -8,6 +8,7 @@ This is the home for Kiwix apps for Apple iOS and macOS.
 [![Codecov](https://codecov.io/gh/kiwix/kiwix-apple/branch/main/graph/badge.svg)](https://codecov.io/gh/kiwix/kiwix-apple)
 [![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 <img src="https://img.shields.io/badge/Swift-5.9-orange.svg" alt="Drawing="/>
+[![Join Slack](https://img.shields.io/badge/Join%20us%20on%20Slack%20%23kiwix--apple-2EB67D)](https://slack.kiwix.org)
 
 ## Download