Z:gnu-www-ja-gpl-faq--ee001a-You should put a notice at the/en

You should put a notice at the start of each source file, stating what license it carries, in order to avoid risk of the code's getting disconnected from its license. If your repository's README says that source file is under the GNU GPL, what happens if someone copies that file to another program? That other context may not show what the file's license is. It may appear to have some other license, or no license at all (which would make the code nonfree).