docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # loopback-component-jsonapi
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/digitalsadhu/loopback-component-jsonapi.svg)](https://greenkeeper.io/)
+
 [![Join the chat at https://gitter.im/digitalsadhu/loopback-component-jsonapi](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/digitalsadhu/loopback-component-jsonapi?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://travis-ci.org/digitalsadhu/loopback-component-jsonapi.svg)](https://travis-ci.org/digitalsadhu/loopback-component-jsonapi)
 [![npm version](https://badge.fury.io/js/loopback-component-jsonapi.svg)](http://badge.fury.io/js/loopback-component-jsonapi)