docs(contributing): replace npm install with yarn

Related to vuejs#9831

Author: posva

.github/CONTRIBUTING.md

@@ -35,12 +35,12 @@ Hi! I'm really excited that you are interested in contributing to Vue.js. Before
 
 ## Development Setup
 
-You will need [Node.js](http://nodejs.org) **version 6+** and [Java Runtime Environment](http://www.oracle.com/technetwork/java/javase/downloads/index.html) (for running Selenium server during e2e tests).
+You will need [Node.js](http://nodejs.org) **version 8+** and [Java Runtime Environment](http://www.oracle.com/technetwork/java/javase/downloads/index.html) (for running Selenium server during e2e tests).
 
 After cloning the repo, run:
 
 ``` bash
-$ npm install # or yarn
+$ yarn # it install the dependencies of the project
 ```
 
 ### Committing Changes