Add CC0 as license

[akirataguchi115]

In 2009, Creative Commons released CC0, which was created for compatibility with jurisdictions where dedicating to public domain is problematic, such as continental Europe. This is achieved by a public-domain waiver statement and a fall-back all-permissive license, for cases where the waiver is not valid.[5][6] The Free Software Foundation[7][8] and the Open Knowledge Foundation approved CC0 as a recommended license to dedicate content to the public domain

i.e. Since a dedication to public domain is not valid in majority of countries outside the U.S. a license like CC0 would be the closest legally valid dedication to public domain.