README.md: Fix legal error - CC0 is not a license
README.md: Fix legal error - CC0 is not a license

Public domain frees the code from copyright, so there is no power of licensing over it anymore, and it is not needed )
mccolgst/www.gittip.com,gratipay/gratipay.com,mccolgst/www.gittip.com,mccolgst/www.gittip.com,gratipay/gratipay.com,eXcomm/gratipay.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,eXcomm/gratipay.com