Adds information about asset-packagist in installation guide.
Adds information about asset-packagist in installation guide.

closes #261

dmitry-kulikov/yii2-app-advanced,yiisoft/yii2-app-advanced,dmitry-kulikov/yii2-app-advanced,erycamel/yii2-app-advanced,githubjeka/yii2-app-advanced,yiijan/yii2-app-advanced,yiijan/yii2-app-advanced,yiisoft/yii2-app-advanced,githubjeka/yii2-app-advanced,erycamel/yii2-app-advanced