Add CI status

Author: Jiaxin Fan

docs/users/standalone/README.md

@@ -19,6 +19,7 @@
 
 # Tests for OpenWhisk NodeJS Runtime as Standalone Container
 [![License](https://img.shields.io/badge/license-Apache--2.0-blue.svg)](http://www.apache.org/licenses/LICENSE-2.0)
+[![Continuous Integration](https://github.com/apache/openwhisk-runtime-nodejs/actions/workflows/ci.yaml/badge.svg)](https://github.com/apache/openwhisk-runtime-nodejs/actions/workflows/ci.yaml)
 
 This README walks you through how to build, customise and test Apache OpenWhisk Node.js runtime images.
 ## Pre-requisites