be more clear about migrations

tests/README.md

@@ -27,7 +27,7 @@ After creating the basic application, follow these steps to prepare for the test
    composer require --dev yiisoft/yii2-faker:*
    ```
 
-3. Create `yii2_basic_tests` database and update it by applying migrations:
+3. Create `yii2_basic_tests` database and update it by applying migrations (you may skip this step if you do not have created any migrations yet):
 
    ```
    codeception/bin/yii migrate