docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # developer.github.com
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/jackass0528/developer.github.com.svg)](https://greenkeeper.io/)
+
 This is a GitHub API resource built with [Nanoc][nanoc].
 
 All submissions are welcome. To submit a change, fork this repo, commit your changes, and send us a [pull request](http://help.github.com/send-pull-requests/).