Add template for PR on github

.github/PULL_REQUEST_TEMPLATE.md

+This project is now tracked on our hosted gitlab server at:
+> https://git.duniter.org/clients/cesium/cesium
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests proposed on github would generate more work for the main contributors, therefore we would really appreciate if you can create instead merge requests on our hosted gitlab instance.
+
+Thank you for your understanding.