Suggest to use .env for enabling polling mode (facebook#1698)

packages/react-scripts/template/README.md

@@ -1530,7 +1530,7 @@ If this doesn’t happen, try one of the following workarounds:
 * Some editors like Vim and IntelliJ have a “safe write” feature that currently breaks the watcher. You will need to disable it. Follow the instructions in [“Working with editors supporting safe write”](https://webpack.github.io/docs/webpack-dev-server.html#working-with-editors-ides-supporting-safe-write).
 * If your project path contains parentheses, try moving the project to a path without them. This is caused by a [Webpack watcher bug](https://github.com/webpack/watchpack/issues/42).
 * On Linux and macOS, you might need to [tweak system settings](https://webpack.github.io/docs/troubleshooting.html#not-enough-watchers) to allow more watchers.
-* If the project runs inside a virtual machine such as (a Vagrant provisioned) VirtualBox, run `npm install --save-dev cross-env` in its root folder and then replace `"react-scripts start"` in the `scripts` section of its `package.json` with `"cross-env CHOKIDAR_USEPOLLING=true react-scripts start"`. This ensures that the next time you run `npm start`, the watcher uses the polling mode, as necessary inside a VM.
+* If the project runs inside a virtual machine such as (a Vagrant provisioned) VirtualBox, create an `.env` file in your project directory if it doesn’t exist, and add `CHOKIDAR_USEPOLLING=true` to it. This ensures that the next time you run `npm start`, the watcher uses the polling mode, as necessary inside a VM.
 
 If none of these solutions help please leave a comment [in this thread](https://github.com/facebookincubator/create-react-app/issues/659).