Placing the license in COPYING.txt forces some tools to add manual overrides:

Packages detected with unsafe licenses:
{"name":"tweetnacl@0.14.3","repository":"git+https://github.com/dchest/tweetnacl-js","license":"SEE LICENSE IN COPYING.txt","source":"package.json"}

It would be better, and more standard, to have the Public Domain text listed in LICENSE.md. GitHub will even do it for you now.