Add PHPSandbox to IDE section (#1152)

README.md

 * [Apache NetBeans](https://netbeans.apache.org/) - An IDE with support for PHP and HTML5.
 * [PhpStorm](https://www.jetbrains.com/phpstorm/) - A commercial PHP IDE.
 * [VS Code](https://code.visualstudio.com/) - An open source code editor.
+* [PHPSandbox](https://phpsandbox.io) - An online IDE for PHP in the browser.
 
 ### Web Applications
 *Web-based applications and tools.*