add Qt Publisher Guide, require agreement to it in pull request template.

Author: lavonardo-qt

.github/pull_request_template.md

@@ -14,3 +14,4 @@ This may include:
 - [ ] I have run `npm run all` to validate my changes
 - [ ] I have made sure my commits are [signed](https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits)
 - [ ] I have added an entry for new Extensions in [CODEOWNERS](/CODEOWNERS)
+- [ ] I have read and I agree with the [Qt Creator Extensions Store Publisher Terms](https://github.com/qt-creator/extension-registry/documentation/publisher-terms.pdf)