docs(readme): add Greenkeeper badge

greenkeeper[bot] · greenkeeper[bot] · commit bb32b2cb9122 · 2018-04-23T20:39:15.000Z
diff --git a/README.md b/README.md
@@ -4,6 +4,8 @@
 
 ## Installation ##
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/continuous-software/node-payflow.svg)](https://greenkeeper.io/)
+
     $ npm install -s payflow
 
 ## Usage
