Add the CC0 license to properly uncopyright code

[ghost]

The current file in the repo actually leaves the code copyrighted. This PR amends the issue.

[ghost]

@ncase I currently have PRs to resolve this issue for this repo and three others:

• https://github.com/ncase/polygons/pull/31
• https://github.com/ncase/loopy/pull/11
• https://github.com/ncase/neurons/pull/6

[ncase]

Whoops, sorry I left this hanging, thanks again @rogersachan!