docs added

Added docs. @cebe since you moved almost everything to `require` section, i dont know how correctly explain user that he need to install first `yiisoft/yii2-codeception`, so i made small note about it in `README.md` under `tests` folder. Not sure if this is right - removing `debug/gii/codeception` from basic boilerplate, but anyway added note about it in main `README.md` in basic application.