Add yarn steps for adding flow (facebook#1756)

[skip ci]

Author: zertosh

packages/react-scripts/template/README.md

@@ -630,12 +630,12 @@ Recent versions of [Flow](http://flowtype.org/) work with Create React App proje
 
 To add Flow to a Create React App project, follow these steps:
 
-1. Run `npm install --save-dev flow-bin`.
+1. Run `npm install --save-dev flow-bin` (or `yarn add --dev flow-bin`).
 2. Add `"flow": "flow"` to the `scripts` section of your `package.json`.
-3. Run `npm run flow -- init` to create a [`.flowconfig` file](https://flowtype.org/docs/advanced-configuration.html) in the root directory.
+3. Run `npm run flow -- init` (or `yarn flow -- init`) to create a [`.flowconfig` file](https://flowtype.org/docs/advanced-configuration.html) in the root directory.
 4. Add `// @flow` to any files you want to type check (for example, to `src/App.js`).
 
-Now you can run `npm run flow` to check the files for type errors.
+Now you can run `npm run flow` (or `yarn flow`) to check the files for type errors.
 You can optionally use an IDE like [Nuclide](https://nuclide.io/docs/languages/flow/) for a better integrated experience.
 In the future we plan to integrate it into Create React App even more closely.