[real-world uses] Add interactive SICP link

doc/realworld.html

@@ -138,6 +138,7 @@
       <li><a href="http://snippets.pro/">Snippets.pro</a> (code snippet sharing)</li>
       <li><a href="http://www.solidshops.com/">SolidShops</a> (hosted e-commerce platform)</li>
       <li><a href="http://sqlfiddle.com">SQLFiddle</a> (SQL playground)</li>
+      <li><a href="http://xuanji.appspot.com/isicp/">Structure and Interpretation of Computer Programs</a>, Interactive Version</li>
       <li><a href="http://syframework.alwaysdata.net">SyBox</a> (PHP playground)</li>
       <li><a href="http://www.tagspaces.org/">TagSpaces</a> (personal data manager)</li>
       <li><a href="https://thefiletree.com">The File Tree</a> (collab editor)</li>