add maintainer-specific template

Author: 2bndy5

.github/ISSUE_TEMPLATE/maintainers-only.md

@@ -0,0 +1,12 @@
+---
+name: Maintainers' note
+about: ideas related to maintaining the cpp-linter-action source code
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+This template is intended only for maintainers of cpp-linter org.
+
+Only use this template if your query is **not** related to a problem or feature.