Add license link to readme.

Author: t-paul

README.md

@@ -1,2 +1,5 @@
 # openscad-documentation
 OpenSCAD offline documentation based on books published via Wikibooks
+
+---
+[![Creative Commons License](https://i.creativecommons.org/l/by-sa/3.0/88x31.png)](http://creativecommons.org/licenses/by-sa/3.0/) - This work is licensed under a [Creative Commons Attribution-ShareAlike 3.0 Unported License](http://creativecommons.org/licenses/by-sa/3.0/).