Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -7,6 +7,8 @@ Netron can run as both an [Electron](https://electronjs.org) app or a web app.
 To start the Electron app, install [Node.js](https://nodejs.org) and run: 
 
 ```bash
+git clone https://github.com/lutzroeder/netron
+cd netron
 npm install
 npm start
 ```