docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # Typed Text Buffer  [![Build Status](https://travis-ci.org/typed-typings/npm-text-buffer.svg?branch=master)](https://travis-ci.org/typed-typings/npm-text-buffer)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/types/npm-text-buffer.svg)](https://greenkeeper.io/)
+
 The type definition for [`text-buffer`](https://github.com/atom/text-buffer.git)
 
 This is not complete. I only implement those that is needed for `env-atom`. Contribution welcome!