Added pull request template

Show a reminder about the special QR branches.

.github/pull_request_template.md

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+
+# Notes
+
+## SLE Quarterly Updates
+
+The SLE15 media are quarterly rebuilt with the latest updated packages. These
+media have the self-update mechanism disabled for several reasons. That is
+implemented in separate `SLE-15-SP*-QR` branches.
+
+*Whenever you update any `SLE-15-SP*` branch do not forget to merge the change
+also to the respective `SLE-15-SP*-QR` branch if it exists.*
+
+After merging the change submit the package manually using the `rake osc:sr`
+command. (It submits to a special quarterly update project, not to the standard
+maintenance project as usually.)