readme: add instructions for running locally

fixes #1311

Author: fippo

README.md

@@ -2,6 +2,13 @@
 
 This is a repository for the WebRTC JavaScript code samples. All of the samples can be tested from [webrtc.github.io/samples](https://webrtc.github.io/samples).
 
+To run the samples locally
+```
+npm install && npm start
+```
+and open your browser on the page indicated.
+
+## Contributing
 We welcome contributions and bugfixes. Please see [CONTRIBUTING.md](https://github.com/webrtc/samples/blob/gh-pages/CONTRIBUTING.md) for details.
 
 ## Bugs