readme: add e2e badge

Author: wikkyk

README.md

@@ -1,6 +1,7 @@
 # Kubernetes Cluster API Provider for Proxmox Virtual Environment - CAPMOX
 
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=ionos-cloud_cluster-api-provider-proxmox&metric=alert_status&token=fb1b4c0a87d83a780c76c21be0f89dc13efc2ca0)](https://sonarcloud.io/summary/new_code?id=ionos-cloud_cluster-api-provider-proxmox)
+[![End-to-End Test Status](https://github.com/ionos-cloud/cluster-api-provider-proxmox/actions/workflows/e2e.yml/badge.svg?branch=main)](https://github.com/ionos-cloud/cluster-api-provider-proxmox/actions/workflows/e2e.yml?query=branch%3Amain)
 
 ## Overview