docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

README.md

@@ -1,4 +1,6 @@
 # Parse SDK for JavaScript
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/parse-community/Parse-SDK-JS.svg)](https://greenkeeper.io/)
 [![Build Status][build-status-svg]][build-status-link]
 [![Test Coverage][coverage-status-svg]][coverage-status-link]
 [![Npm Version][npm-svg]][npm-link]