title: Wordpress: How does 'Insert Read More tag' works?
body: <p>I want to add a button to tinyMCE editor on new post page. With <a href="http://code.tutsplus.com/tutorials/guide-to-creating-your-own-wordpress-editor-buttons--wp-30182" rel="noreferrer">this toturial</a> I managed to get the button work perfectly, but there is something I couldn't figure out. When you insert a "More" tag, an image will appended to the html with appropriate 'background-image'. See screenshot bellow:
  <img src="https://i.stack.imgur.com/QsA27.png" alt="more tag"></p>
  
  <p>But when you switch to 'Text' mode there is a html comment like this: <code>&lt;!--more--&gt;</code>.
  <img src="https://i.stack.imgur.com/TJHfg.png" alt="enter image description here"></p>
  
  <p>I could add the image in the html but on 'Text' mode there is an <code>img</code> tag.
  <img src="https://i.stack.imgur.com/qgWN7.png" alt="enter image description here">
  <img src="https://i.stack.imgur.com/rExaj.png" alt="enter image description here"></p>
  
  <p>I want to have something like this: <code>&lt;!--my-custom-tag--&gt;</code></p>
  
  <p>How wordpress manage to do this? Or how could I append a custom tag on tinyMCE editor?</p>