docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,6 +1,6 @@
 # std
 
-[![All Contributors](https://img.shields.io/badge/all_contributors-4-orange.svg?style=flat-square)](#contributors)
+[![All Contributors](https://img.shields.io/badge/all_contributors-4-orange.svg?style=flat-square)](#contributors) [![Greenkeeper badge](https://badges.greenkeeper.io/sandstreamdev/std.svg)](https://greenkeeper.io/)
 
 ## Contributors ✨