change: add explanation of aurweb vs user packages in Bug.md

Signed-off-by: Kevin Morris <kevr@0cost.org>

.gitlab/issue_templates/Bug.md

+### Checklist
+
+This bug template is meant to provide bug issues for code existing in
+the aurweb repository. This bug template is **not meant** to handle
+bugs with user-uploaded packages.
+
+To work out a bug you have found in a user-uploaded package, contact
+the package's maintainer first. If you receive no response, file the
+relevent package request against it so TUs can deal with cleanup.
+
+- [ ] I confirm that this is an issue with aurweb's code and not a
+      user-uploaded package.
 - [ ] I have described the bug in complete detail in the
       [Description](#description) section.
 - [ ] I have specified steps in the [Reproduction](#reproduction) section.