add CC0 license

The rationale here is we don't believe YAML files contained in this repository are actually copyrightable. They are more like math than code, really, so let's not bother anyone with this crap.

It's also very unlikely people will reuse entire source files as is, which makes copyright claims even less credible.

On the other hand, if we do leave copyright more open, we're possibly more likely to have others reuse those formulas and, maybe, who knows, contribute back.

Source: https://creativecommons.org/publicdomain/zero/1.0/legalcode.txt Inspiration: tpo/network-health/metrics/timeline@9d7cfeeb Closes: #10 (closed)