Add link to GPLv4 badge

README.rst

@@ -8,6 +8,7 @@ CAS Server
     :target: https://pypi.python.org/pypi/django-cas-server
 
 .. image:: https://img.shields.io/pypi/l/django-cas-server.svg
+    :target: https://www.gnu.org/licenses/gpl-3.0.html
 
 CAS Server is a Django application implementing the `CAS Protocol 3.0 Specification
 <https://jasig.github.io/cas/development/protocol/CAS-Protocol-Specification.html>`_.