[real-world uses] Add SageMathCell

doc/realworld.html

@@ -142,6 +142,7 @@
       <li><a href="http://rascalmicro.com/docs/basic-tutorial-getting-started.html">Rascal</a> (tiny computer)</li>
       <li><a href="https://www.realtime.io/">RealTime.io</a> (Internet-of-Things infrastructure)</li>
       <li><a href="http://refork.com/">Refork</a> (animation demo gallery and sharing)</li>
+      <li><a href="http://sagecell.sagemath.org">SageMathCell</a> (interactive mathematical software)</li>
       <li><a href="https://cloud.sagemath.com/">SageMathCloud</a> (interactive mathematical software environment)</li>
       <li><a href="https://chrome.google.com/webstore/detail/servephp/mnpikomdchjhkhbhmbboehfdjkobbfpo">ServePHP</a> (PHP code testing in Chrome dev tools)</li>
       <li><a href="https://www.shadertoy.com/">Shadertoy</a> (shader sharing)</li>