docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -2,7 +2,7 @@
 
 [![npm version](https://img.shields.io/npm/v/typescript-test-utils.svg "test")](https://www.npmjs.com/package/typescript-test-utils)
 [![Conventional Commits](https://img.shields.io/badge/Conventional%20Commits-1.0.0-yellow.svg)](https://conventionalcommits.org)
-[![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg?style=flat-square)](https://github.com/prettier/prettier)
+[![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg?style=flat-square)](https://github.com/prettier/prettier) [![Greenkeeper badge](https://badges.greenkeeper.io/LeDDGroup/typescript-test-utils.svg)](https://greenkeeper.io/)
 
 Helper types for testing your package exported types