- add a tip that rbac.php is needed to create manually when using PhpManager.
- update that RbacController example needs to import Yii namespace.

Change git clone git@github.com:yiisoft/yii2-app-basic.git to 
git clone https://github.com/yiisoft/yii2-app-basic

Fixed typo in validation guide

We need an image of the CRUD preview (or we need to change the text).

Not clear how define('YII_ENV', 'dev') creates the YII_ENV_DEV
constant. Seems like an odd bit of magic that may confuse others…

Should it be “quality of these articles may not be as good”?