message: Update contributing docs to use yarn
diff: @@ -28,31 +28,37 @@ The core team will be monitoring for pull requests. When we get one, we'll run s
  git checkout -b my_branch
  ```
  -2. Run `npm install`. We recommend that you use `npm` version 3 or later.
  +2. Jest uses [Yarn](https://code.facebook.com/posts/1840075619545360)
  + for running development scripts. If you haven't already done so,
  + please [install yarn](https://yarnpkg.com/en/docs/install).
  +
  +3. Run `yarn install`.
  ```sh
  - npm install
  + yarn install
  ```
  -3. If you've added code that should be tested, add tests. You
  +4. If you've added code that should be tested, add tests. You
  can use watch mode that continuously transforms changed files
  to make your life easier.
  ```sh
  # in the background
  - npm run watch
  + yarn run watch
  ```
  -4. If you've changed APIs, update the documentation.
  -5. Ensure the test suite passes via `npm test`. To run the test suite you
  +5. If you've changed APIs, update the documentation.
  +
  +6. Ensure the test suite passes via `yarn test`. To run the test suite you
  may need to install Mercurial (`hg`). On macOS, this can be done
  using [homebrew](http://brew.sh/): `brew install hg`.
  ```sh
  brew install hg # maybe
  - npm test
  + yarn test
  ```
  -6. If you haven't already, complete the CLA.
  +
  +7. If you haven't already, complete the CLA.
  ### Contributor License Agreement (CLA)
  @@ -66,7 +72,7 @@ To link `jest` on the command line to `jest-cli/bin/jest.js` in a development bu
  ```sh
  cd /path/to/your/Jest_clone/packages/jest-cli
  -npm link
  +yarn link
  ```
  To build Jest:
  @@ -77,10 +83,10 @@ cd /path/to/your/Jest_clone
  # Do one of the following:
  # Check out a commit from another contributor, and then
  -npm run build
  +yarn run build
  -# Save your changes to Jest, and then
  -npm test # which also builds Jest
  +# Or, save your changes to Jest, and then
  +yarn test # which also builds Jest
  ```
  To run tests in another project with the development build of Jest:
  @@ -95,7 +101,7 @@ jest [options] # run jest-cli/bin/jest.js in the development build
  To unlink `jest` on the command line from `jest-cli/bin/jest.js` in a development build:
  ```sh
  -npm unlink --global jest-cli
  +yarn unlink jest-cli
  ```
  ## Bugs
  