title: What is package-info.java and how can I add it to my project?
body: <p>When I run Checkstyle over my Spring project it says:</p>
  
  <pre><code>- Missing package-info.java file.
  </code></pre>
  
  <ol>
  <li>what is <strong><em>package-info.java</em></strong> and how is it useful? </li>
  <li>how can I deal with this message?</li>
  </ol>