docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # gulp-header-comment
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/mjeanroy/gulp-header-comment.svg)](https://greenkeeper.io/)
+
 [![Build Status](https://travis-ci.org/mjeanroy/gulp-header-comment.svg?branch=master)](https://travis-ci.org/mjeanroy/gulp-header-comment)
 [![Npm version](https://badge.fury.io/js/gulp-header-comment.svg)](https://badge.fury.io/js/gulp-header-comment)