docs(readme): add Greenkeeper badge

greenkeeper[bot] · greenkeeper[bot] · commit f66b7f89fbe0 · 2017-10-18T08:57:09.000Z
diff --git a/README.md b/README.md
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 # JSON Pointer for nodejs
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/janl/node-jsonpointer.svg)](https://greenkeeper.io/)
+
 This is an implementation of [JSON Pointer](http://tools.ietf.org/html/draft-ietf-appsawg-json-pointer-08).
 
 ## Usage
