Add a README for eslint-config-react-app

packages/eslint-config-react-app/README.md

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+# eslint-config-react-app
+
+This package includes the shareable ESLint configuration used by Create React App.
+
+## Installation
+
+*The easiest way to use this configuration is with *[Create React App](https://github.com/facebookincubator/create-react-app)*, which includes it by default – you **don't need to install it separately in Create React App projects.***
+
+If you want to use this ESLint configuration in a project not built with Create React App, you can install it with following steps.
+
+1. Install this package, ESLint and the necessary plugins:
+  ```
+  npm install [email protected] [email protected] eslint-plugin-flowtype2.18.1 [email protected] [email protected] [email protected] --save-dev
+  ```
+2. Create a file named `.eslintrc` with following contents in the root folder of your project:
+  ```
+  {
+    "extends": "react-app"
+  }
+  ```
+  That's it! You can override the settings from `eslint-config-react-app` by editing the `.eslintrc` file. Learn more about [configuring ESLint](http://eslint.org/docs/user-guide/configuring) on the ESLint website.