title: Error when installed Yii2
body: <p>I've install Yii2 framework using composer but get this error in my browser (on localhost):</p>
  <blockquote>
  <p>Invalid Configuration – yii\base\InvalidConfigException</p>
  <p>yii\web\Request::cookieValidationKey must be configured with a secret key.</p>
  </blockquote>
  <p>How can I solve this problem?</p>