README.md: add slack badge

Signed-off-by: leonnicolas <leonloechner@gmx.de>

README.md

@@ -7,6 +7,7 @@ Kilo is a multi-cloud network overlay built on WireGuard and designed for Kubern
 [![Build Status](https://github.com/squat/kilo/workflows/CI/badge.svg)](https://github.com/squat/kilo/actions?query=workflow%3ACI)
 [![Go Report Card](https://goreportcard.com/badge/github.com/squat/kilo)](https://goreportcard.com/report/github.com/squat/kilo)
 [![Docker Pulls](https://img.shields.io/docker/pulls/squat/kilo)](https://hub.docker.com/r/squat/kilo)
+[![Slack](https://img.shields.io/badge/join%20slack-%23kilo-brightgreen.svg)](https://slack.k8s.io/)
 
 ## Overview