Added xkcd comic and license

The xkcd comic is in the extern folder together with the license for the file.
See issue #22

Author: weerox

extern/xkcd/README.md

@@ -0,0 +1,8 @@
+These images has been copied from xkcd.com
+
+- https://imgs.xkcd.com/comics/regular_expressions.png
+- https://www.xkcd.com/208/
+
+License:
+- https://creativecommons.org/licenses/by-nc/2.5/
+- This file is licensed under Creative Commons Attribution-NonCommercial 2.5 Generic (CC BY-NC 2.5) License. https://creativecommons.org/licenses/by-nc/2.5/

extern/xkcd/regular-expressions.png

@@ -10,7 +10,9 @@
   test if the input of a program is correct, and check if the users made mistakes.
 </p>
 <p>
-  <a target="_blank" href="https://www.xkcd.com/208/"><img src="https://imgs.xkcd.com/comics/regular_expressions.png"></a>
+  <a target="_blank" href="../extern/xkcd/README.md">
+    <img src="../extern/xkcd/regular-expressions.png">
+  </a>
 </p>
 <script type="text/javascript">
   window.addEventListener("load", function(){