[minor] Add license picture, as in LICENSE

README.md

@@ -10,4 +10,9 @@ The original JAXON description was published under the CC BY-SA 4.0 license in t
 
 ## License
 
-This robot description is distributed under the [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/).
+This robot description is licensed under the [Creative Commons Attribution-ShareAlike 4.0 International License][cc-by-sa].
+
+[![CC BY-SA 4.0][cc-by-sa-image]][cc-by-sa]
+
+[cc-by-sa]: http://creativecommons.org/licenses/by-sa/4.0/
+[cc-by-sa-image]: https://licensebuttons.net/l/by-sa/4.0/88x31.png