Add READMEs for packages

packages/babel-preset-react-app/README.md

 # babel-preset-react-app
 
-This package includes the Babel preset used by [Create React App](https://github.com/facebookincubator/create-react-app).
+This package includes the Babel preset used by [Create React App](https://github.com/facebookincubator/create-react-app).  
+Please refer to its documentation:
+
+* [Getting Started](https://github.com/facebookincubator/create-react-app/blob/master/README.md#getting-started) – How to create a new app.
+* [User Guide](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md) – How to develop apps bootstrapped with Create React App.
 
 ## Usage in Create React App Projects
 

packages/create-react-app/README.md

+# create-react-app
+
+This package includes the global command for [Create React App](https://github.com/facebookincubator/create-react-app).  
+Please refer to its documentation:
+
+* [Getting Started](https://github.com/facebookincubator/create-react-app/blob/master/README.md#getting-started) – How to create a new app.
+* [User Guide](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md) – How to develop apps bootstrapped with Create React App.

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

 # eslint-config-react-app
 
-This package includes the shareable ESLint configuration used by [Create React App](https://github.com/facebookincubator/create-react-app).
+This package includes the shareable ESLint configuration used by [Create React App](https://github.com/facebookincubator/create-react-app).  
+Please refer to its documentation:
+
+* [Getting Started](https://github.com/facebookincubator/create-react-app/blob/master/README.md#getting-started) – How to create a new app.
+* [User Guide](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md) – How to develop apps bootstrapped with Create React App.
 
 ## Usage in Create React App Projects
 

packages/react-scripts/README.md

+# react-scripts
+
+This package includes scripts and configuration used by [Create React App](https://github.com/facebookincubator/create-react-app).  
+Please refer to its documentation:
+
+* [Getting Started](https://github.com/facebookincubator/create-react-app/blob/master/README.md#getting-started) – How to create a new app.
+* [User Guide](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md) – How to develop apps bootstrapped with Create React App.