add GitHub pull request template

.github/PULL_REQUEST_TEMPLATE.md

+*This repository has moved, PLEASE do not open a pull request here!*
+
+You can, instead, open a pull request on the self-hosted GitLab instance at: https://repo.getmonero.org/monero-project/monero-site
+
+- [ ] I have read the above message
+- [ ] Despite reading the above I've ignored it
+- [ ] I understand that if I open a PR it'll just be closed
+- [ ] But I'm a total idiot
+- [ ] Because of the above I'm opening a pull request anyway