added join slack badge to readme (kubernetes-sigs#1218)

Signed-off-by: Nir Rozenbaum <[email protected]>

Author: nirrozenbaum

README.md

@@ -1,6 +1,7 @@
 [![Go Report Card](https://goreportcard.com/badge/sigs.k8s.io/gateway-api-inference-extension)](https://goreportcard.com/report/sigs.k8s.io/gateway-api-inference-extension)
 [![Go Reference](https://pkg.go.dev/badge/sigs.k8s.io/gateway-api-inference-extension.svg)](https://pkg.go.dev/sigs.k8s.io/gateway-api-inference-extension)
 [![License](https://img.shields.io/github/license/kubernetes-sigs/gateway-api-inference-extension)](/LICENSE)
+[![Join Slack](https://img.shields.io/badge/Join_Slack-blue?logo=slack)](https://kubernetes.slack.com/archives/C08E3RZMT2P)
 
 # Gateway API Inference Extension