github: add a basic issue template

As documented here:

https://help.github.com/articles/creating-an-issue-template-for-your-repository/

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,29 @@
+### Submission type
+
+[ ] Bug report
+
+[ ] Request for enhancement (RFE)
+
+*NOTE: Do not submit anything other than bug reports or RFEs via the issue tracker!*
+
+### systemd version the issue has been seen with
+
+> …
+
+*NOTE: Do not submit bug reports about anything but the two most recently released systemd versions upstream!*
+
+### Used distribution
+
+> …
+
+### In case of bug report: Expected behaviour you didn't see
+
+> …
+
+### In case of bug report: Unexpected behaviour you saw
+
+> …
+
+### In case of bug report: Steps to reproduce the problem
+
+> …