docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # fast-json-stringify  [![Build Status](https://travis-ci.org/fastify/fast-json-stringify.svg?branch=master)](https://travis-ci.org/fastify/fast-json-stringify)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/fastify/fast-json-stringify.svg)](https://greenkeeper.io/)
+
 __fast-json-stringify__ is x1-4 times faster than `JSON.stringify()`.
 It is particularly suited if you are sending small JSON payloads, the
 advantages reduces on large payloads.