Added back CircleCI badge to README.md (#132)

Author: Josh Goldberg

README.md

@@ -4,6 +4,7 @@
 ![Coverage: 100%](https://img.shields.io/badge/coverage-100%25-orange.svg)
 ![TypeScript: Strict](https://img.shields.io/badge/typescript-strict-yellow.svg)
 [![NPM version](https://badge.fury.io/js/tslint-to-eslint-config.svg)](http://badge.fury.io/js/tslint-to-eslint-config)
+[![Circle CI](https://img.shields.io/circleci/build/github/typescript-eslint/tslint-to-eslint-config.svg)](https://circleci.com/gh/typescript-eslint/tslint-to-eslint-config)	
 [![Join the chat at https://gitter.im/tslint-to-eslint-config/community](https://img.shields.io/badge/chat-gitter-informational.svg)](https://gitter.im/tslint-to-eslint-config/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Code Style: Prettier](https://img.shields.io/badge/speed-blazingly_fast-blueviolet.svg)](https://prettier.io)