Add back build badge in README.md

Author: tngan

README.md

@@ -1,5 +1,6 @@
 # samlify ·
 
+[![Build Status](https://img.shields.io/circleci/build/github/tngan/samlify?style=for-the-badge&logo=circleci)](https://app.circleci.com/pipelines/github/tngan/samlify)
 [![npm version](https://img.shields.io/npm/v/samlify.svg?style=for-the-badge&logo=npm)](https://www.npmjs.com/package/samlify)
 [![NPM](https://img.shields.io/npm/dm/samlify.svg?style=for-the-badge&logo=npm)](https://www.npmjs.com/package/samlify)
 [![Coverage Status](https://img.shields.io/coveralls/tngan/samlify/master.svg?style=for-the-badge&logo=coveralls)](https://coveralls.io/github/tngan/samlify?branch=master)