Apply licensing properly #1
Description
According to https://www.gnu.org/licenses/gpl-howto.html.en each file should have is own license header / copyright notice.
In order to completely fulfill the license and correctly protect and address your software, it should be better to have a copyright header in each file, also stating licenses for READMEs and other additional files.
A very useful tool to use for checking your license is, for example REUSE, by FSF Europe (https://reuse.software): by installing and using its linter through the simple command reuse lint, it'll show the files with missing licenses. It'll also allow to automatically download all applied licenses.