github: Add issue template to help people find upstream

.github/ISSUE_TEMPLATE/report-an-issue.md

@@ -0,0 +1,15 @@
+---
+name: Report an issue
+about: Any issue relating to build/sync of this fork
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+As noted in the README.md, this fork of
+`paradoxxxzero/gnome-shell-system-monitor-applet` is for packaging purposes
+only. Any issues, bug reports, feature requests, etc. for the extension
+itself should be submitted to the [upstream
+repo](https://github.com/paradoxxxzero/gnome-shell-system-monitor-applet),
+but build/sync issues should be reported here.