docs: update PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

@@ -12,5 +12,6 @@ Thank you for contributing to natrontech/pbs-exporter.
 **Checklist**:
 
 - [ ] I have read and understood the [CONTRIBUTING](https://github.com/natrontech/pbs-exporter/blob/main/CONTRIBUTING.md) and [CODE_OF_CONDUCT](https://github.com/natrontech/pbs-exporter/blob/main/CODE_OF_CONDUCT.md) documents
+- [ ] All commits are signed (see [Signing Commits](https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification))
 - [ ] Pull Request title in the format of [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) e.g. `feat|fix|chore|docs|...: Changed Something`
 - [ ] Updated documentation in the  `README.md` file (e.g. new parameters, environment variables, return values, ...)