Add template for pull requests at GitHub for mirroring

.github/PULL_REQUEST_TEMPLATE.md

+<!-- AlekSIS is developed on EduGit. GitHub only serves as
+     backup mirror and to help people find the project. If
+	 possible, please submit your merge request on EduGit!
+	 
+	 EduGit accepts logins with GitHub accounts.
+-->
+
+[ ] I have read the above and have no way to contribute on EduGit
+[ ] I understand that GitHub's terms of service exclude young and 
+    learning contributors, but still cannot contribute on EduGit
+    instead.