Add Codepen to real-world uses

Author: marijnh

doc/realworld.html

@@ -27,6 +27,7 @@ <h1><span class="logo-braces">{ }</span> <a href="http://codemirror.net/">CodeMi
       <li><a href="http://cargocollective.com/">Cargo Collective</a> (creative publishing platform)</li>
       <li><a href="http://www.codebugapp.com/">Codebug</a> (PHP Xdebug front-end)</li>
       <li><a href="http://code.google.com/p/codemirror2-gwt/">CodeMirror2-GWT</a> (Google Web Toolkit wrapper)</li>
+      <li><a href="http://codepen.io">Codepen</a> (gallery of animations)</li>
       <li><a href="http://codev.it/">Codev</a> (collaborative IDE)</li>
       <li><a href="http://ot.substance.io/demo/">Collaborative CodeMirror demo</a> (CodeMirror + operational transforms)</li>
       <li><a href="http://www.ckwnc.com/">CKWNC</a> (UML editor)</li>