Update README.md

Add optional dependencies

Author: lukeapage

build/README.md

@@ -1,5 +1,5 @@
 [![Build Status](https://travis-ci.org/less/less.js.png?branch=master)](https://travis-ci.org/less/less.js)
- [![Dependencies](https://david-dm.org/less/less.js.png)](https://david-dm.org/less/less.js) [![devDependency Status](https://david-dm.org/less/less.js/dev-status.png)](https://david-dm.org/less/less.js#info=devDependencies)
+ [![Dependencies](https://david-dm.org/less/less.js.png)](https://david-dm.org/less/less.js) [![devDependency Status](https://david-dm.org/less/less.js/dev-status.png)](https://david-dm.org/less/less.js#info=devDependencies) [![optionalDependency Status](https://david-dm.org/less/less.js/optional-status.png)](https://david-dm.org/less/less.js#info=optionalDependencies)
 
 # [Less.js v<%= pkg.version %>](http://lesscss.org)