Add build status badge

added CircleCI build status badge to README

Author: chadwcarlson

README.md

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 # Platform.sh Config Reader (Node.js)
 
+[![CircleCI Status](https://circleci.com/gh/platformsh/config-reader-nodejs.svg?style=shield&circle-token=:circle-token)](https://circleci.com/gh/platformsh/config-reader-nodejs)
+
 This library provides a streamlined and easy to use way to interact with a Platform.sh environment.  It offers utility methods to access routes and relationships more cleanly than reading the raw environment variables yourself.
 
 This library requires Node.js 10 or later.