change back the readme instruction for creating new project.

README.md

@@ -52,7 +52,7 @@ at [getcomposer.org](http://getcomposer.org/doc/00-intro.md#installation-nix).
 You can then install this application template using the following command:
 
 ~~~
-php composer.phar create-project --stability=alpha yiisoft/yii2-app-basic basic
+php composer.phar create-project --stability=dev yiisoft/yii2-app-basic basic
 ~~~
 
 Now you should be able to access the application through the following URL, assuming `basic` is the directory