Improve documentation for using React with Webpack

Fixes #5679

docs/getting-started.md

@@ -34,7 +34,7 @@ $ npm install --save react react-dom babelify babel-preset-react
 $ browserify -t [ babelify --presets [ react ] ] main.js -o bundle.js
 ```
 
-To install React DOM and build your bundle with webpack:  
+To install React DOM and build your bundle with webpack:
 
 ```sh
 $ npm install --save react react-dom babel-preset-react
@@ -45,6 +45,15 @@ $ webpack
 >
 > If you are using ES2015, you will want to also use the `babel-preset-es2015` package.
 
+**Note:** by default, React will be in development mode, which is slower, and not advised for production. To use React in production mode, set the environment variable `NODE_ENV` to `production` (using envify or webpack's DefinePlugin). For example:
+
+```js
+new webpack.DefinePlugin({
+  "process.env": {
+    NODE_ENV: JSON.stringify("production")
+  }
+});
+```
 
 ## Quick Start Without npm