.github: update issue template

Author: tulir

.github/ISSUE_TEMPLATE/issue.md

@@ -3,3 +3,10 @@ name: Open issue
 about: For bug reports and feature requests directly related to whatsmeow
 
 ---
+
+<!--
+Do not open issues for questions!
+Issues are only for bug reports and feature requests.
+
+Questions belong in the Matrix room or on GitHub Discussions.
+-->