docs: Update README.md (kubernetes-client#1538)

update travis-ci build status with Github Actions

Author: WeihanLi

README.md

@@ -1,5 +1,6 @@
 # Kubernetes C# Client
-[![Travis](https://img.shields.io/travis/kubernetes-client/csharp.svg)](https://travis-ci.org/kubernetes-client/csharp)
+
+[![Github Actions Build](https://github.com/kubernetes-client/csharp/actions/workflows/buildtest.yaml/badge.svg)](https://github.com/kubernetes-client/csharp/actions/workflows/buildtest.yaml)
 [![Client Capabilities](https://img.shields.io/badge/Kubernetes%20client-Silver-blue.svg?style=flat&colorB=C0C0C0&colorA=306CE8)](http://bit.ly/kubernetes-client-capabilities-badge)
 [![Client Support Level](https://img.shields.io/badge/kubernetes%20client-beta-green.svg?style=flat&colorA=306CE8)](http://bit.ly/kubernetes-client-support-badge)