docs(readme): add Greenkeeper badge

greenkeeper[bot] · greenkeeper[bot] · commit 19ae46c3b172 · 2017-10-05T14:23:28.000Z
diff --git a/README.md b/README.md
@@ -64,6 +64,8 @@
 
 ## Install
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/tiaanduplessis/codebar-regex.svg)](https://greenkeeper.io/)
+
 ```sh
 $ npm install --save codebar-regex
 # OR
