Change docs license to CC-BY-SA 4.0

After getting the copyright holder Brian Aker's approval, we
are updating thise license to a license that is compatible with
Debian so that the docs can be included. Please see comments in
#11 (comment)
for confirmation.

Closes #11

docs/source/license.rst

@@ -8,9 +8,9 @@ Documentation Content
 
 .. image:: cc-symbol.png
    :alt:  Creative Commons License
-   :target:  http://creativecommons.org/licenses/by-nc-nd/3.0/
+   :target:  http://creativecommons.org/licenses/by-sa/4.0/
 
-Gearman Documentation (gearman.info) is licensed under a `Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License <http://creativecommons.org/licenses/by-nc-nd/3.0/>`_.
+Gearman Documentation (gearman.info) is licensed under a `Attribution-ShareAlike 4.0 International License <http://creativecommons.org/licenses/by-sa/4.0/>`_.
 
 If you need to have the documention licensed in a different manner, please contact `Data Differential <http://www.datadifferential.com/>`_.