title: What does "extra" field mean in composer.json file?
body: <p>I try to add <code>composer.json</code> file to my library. In many examples that I saw I found <code>"extra"</code> field, that look like:</p>
  
  <pre><code>"extra": {
      "branch-alias": {
          "dev-master": "2.0.x-dev"
      }
  }
  </code></pre>
  
  <p>My question is:</p>
  
  <p>Is this a required field?
  What does it mean and what I must write in it?</p>