refactor: Update README with Treehouse link.

README.md

@@ -3,6 +3,12 @@
 
 [![npm](https://img.shields.io/npm/v/d3-dtree.svg)](https://www.npmjs.com/package/d3-dtree) [![Bower](https://img.shields.io/bower/v/d3-dtree.svg)](https://github.com/ErikGartner/dTree) [![Dependency Status](https://david-dm.org/ErikGartner/dtree.svg)](https://david-dm.org/ErikGartner/dtree) [![devDependency Status](https://david-dm.org/ErikGartner/dtree/dev-status.svg)](https://david-dm.org/ErikGartner/dtree#info=devDependencies)
 
+## Treehouse
+There exists a playground/open repository for dTree graphs called [Treehouse](https://treehouse.gartner.io). There anyone can host a dTree graph without having to create a website or interact directly with the library.
+
+Checkout **the demo graph** for dTree:
+https://treehouse.gartner.io/t/dtree-demo
+
 ## Installation
 There are several ways to use dTree. One way is to simply include the compiled file ```dTree.js``` that then exposes a ```dTree``` variable. dTree is available on both NPM and Bower as *d3-dtree*.