message: docs(contributing): add yarn requirement to guide
diff: @@ -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 8+** 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+**, [Java Runtime Environment](http://www.oracle.com/technetwork/java/javase/downloads/index.html) (for running Selenium server during e2e tests) and [yarn](https://yarnpkg.com/en/docs/install).
  After cloning the repo, run:
  ``` bash
  -$ yarn # it install the dependencies of the project
  +$ yarn # install the dependencies of the project
  ```
  ### Committing Changes
  