message: docs(gettingStarted): add Yarn install
  * docs(gettingStarted): add Yarn install
  cc
  * Update GettingStarted.md
diff: @@ -13,6 +13,12 @@ Install Jest using `npm`:
  npm install --save-dev jest
  ```
  +Or via [`yarn`](https://yarnpkg.com/en/package/jest):
  +
  +```
  +yarn add --dev jest
  +```
  +
  Let's get started by writing a test for a hypothetical function that adds two numbers. First, create a `sum.js` file:
  ```javascript
  