add documentation about custom babel configuration

Author: tuchk4

packages/react-scripts/scripts/eject.js

@@ -130,7 +130,7 @@ prompt(
   );
 
   // Add Babel config if there is no local babel config
-  if (isLocalBabelExists()) {
+  if (!isLocalBabelExists()) {
     console.log('  Adding ' + cyan('Babel') + ' preset');
     appPackage.babel = babelConfig;
   }

packages/react-scripts/template/.babelrc

@@ -28,6 +28,7 @@ You can find the most recent version of this guide [here](https://github.com/fac
 - [Using Global Variables](#using-global-variables)
 - [Adding Bootstrap](#adding-bootstrap)
 - [Adding Flow](#adding-flow)
+- [Adding Custom Babel Configuration](#adding-custom-babel-configuration)
 - [Adding Custom Environment Variables](#adding-custom-environment-variables)
 - [Can I Use Decorators?](#can-i-use-decorators)
 - [Integrating with a Node Backend](#integrating-with-a-node-backend)
@@ -494,6 +495,18 @@ In the future we plan to integrate it into Create React App even more closely.
 
 To learn more about Flow, check out [its documentation](https://flowtype.org/).
 
+## Adding Custom Babel Configuration
+
+You can override builtin babel presets and plugins [via .babelrc or package.json](https://babeljs.io/docs/usage/babelrc/).
+
+>Note: Don't forget to install babel-preset-react-app and include "react-app" preset into your .babelrc or package.json babel configuration section.
+
+```js
+{
+  "presets": ["react-app", "stage-0"]
+}
+```
+
 ## Adding Custom Environment Variables
 
 >Note: this feature is available with `react-scripts@0.2.3` and higher.