docs: added REACT_EDITOR to Advanced Configuration section (facebook#2445)

* docs: added REACT_EDITOR to `Advanced Configuration` section

* Update README.md

* Update launchEditor.js

Author: wirmar

packages/react-dev-utils/launchEditor.js

@@ -172,7 +172,8 @@ function printInstructions(fileName, errorMessage) {
       ' to the ' +
       chalk.green('.env.local') +
       ' file in your project folder ' +
-      'and restart the development server.'
+      'and restart the development server. Learn more: ' +
+      chalk.green('https://goo.gl/MMTaZt')
   );
   console.log();
 }

packages/react-scripts/template/README.md

@@ -1826,6 +1826,7 @@ PORT | :white_check_mark: | :x: | By default, the development web server will at
 HTTPS | :white_check_mark: | :x: | When set to `true`, Create React App will run the development server in `https` mode.
 PUBLIC_URL | :x: | :white_check_mark: | Create React App assumes your application is hosted at the serving web server's root or a subpath as specified in [`package.json` (`homepage`)](#building-for-relative-paths). Normally, Create React App ignores the hostname. You may use this variable to force assets to be referenced verbatim to the url you provide (hostname included). This may be particularly useful when using a CDN to host your application.
 CI | :large_orange_diamond: | :white_check_mark: | When set to `true`, Create React App treats warnings as failures in the build. It also makes the test runner non-watching. Most CIs set this flag by default.
+REACT_EDITOR | :white_check_mark: | :x: | When an app crashes in development, you will see an error overlay with clickable stack trace. When you click on it, Create React App will try to determine the editor you are using based on currently running processes, and open the relevant source file. You can [send a pull request to detect your editor of choice](https://github.com/facebookincubator/create-react-app/issues/2636). Setting this environment variable overrides the automatic detection. If you do it, make sure your systems [PATH](https://en.wikipedia.org/wiki/PATH_(variable)) environment variable points to your editor’s bin folder.
 
 ## Troubleshooting