docs(readme): add Greenkeeper badge (#1)

Author: greenkeeper[bot]

README.md

@@ -7,6 +7,7 @@ a more or less human readable AST out of .js or .ts files.
 [![npm](https://img.shields.io/npm/v/typescript-parser.svg?maxAge=3600)](https://www.npmjs.com/package/typescript-parser)
 [![Coverage status](https://img.shields.io/coveralls/TypeScript-Heroes/node-typescript-parser.svg?maxAge=3600)](https://coveralls.io/github/TypeScript-Heroes/node-typescript-parser)
 [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg)](https://github.com/semantic-release/semantic-release)
+[![Greenkeeper badge](https://badges.greenkeeper.io/TypeScript-Heroes/node-typescript-parser.svg)](https://greenkeeper.io/)
 
 ## How to use