add contribution instruction in readme

Author: oopschen

README.md

@@ -133,6 +133,14 @@ module.exports = {
 
 If you want to view the original Sass files inside Chrome and even edit it, [there's a good blog post](https://medium.com/@toolmantim/getting-started-with-css-sourcemaps-and-in-browser-sass-editing-b4daab987fb0). Checkout [test/sourceMap](https://github.com/jtangelder/sass-loader/tree/master/test) for a running example. Make sure to serve the content with an HTTP server.
 
+## Contribution 
+ 
+### How to run test 
+```bash 
+ mkdir -p test/node_modules/ && touch test/node_modules/some-module.s{a,c}ss && touch test/node_modules/underscore.s{a,c}ss
+ npm test
+```
+
 ## License
 
 MIT (http://www.opensource.org/licenses/mit-license.php)