gitlab: Add default issue template

Populate issue descriptions by default with a note directing reporters
to other resources they should consider using first.  Make it a comment
so that the content does not actually render in the report not removed.

.gitlab/issue_templates/Default.md

@@ -0,0 +1,9 @@
+<!--
+This issue tracker is for CMake upstream development:
+
+* If you are having trouble building a specific third-party project
+  that uses CMake, ask for help in that project's forums first.
+
+* If you have a coding or usage question, please ask for help
+  on the CMake discourse forums: https://discourse.cmake.org/
+-->