Update PULL_REQUEST_TEMPLATE.md
Add line with "signed-off"
- [ ] I have read the **CONTRIBUTING** document.
- [ ] I have added tests to cover my changes.
- [ ] All new and existing tests passed.
+
+<!--- It would be nice if you could sign off your contribution by replacing the name with your GitHub user name and GitHub email contact. -->
+Signed-off-by: GITHUB_USER <GITHUB_USER_EMAIL>
