docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # ST Language Support
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/highlightjs/highlightjs-structured-text.svg)](https://greenkeeper.io/)
+
 This repository provide syntax Highlights for highlight.js for Structured Text language. ST is one of the 6 languages of IEC 61131-3 standard developed in 1998 for developing PLC programs.
 
 We want to provide ST syntax highlights in VS Code Markdown editor and Markdown preview. And other cases when tutorials are published in the web.