Add svelte@next caveat to bug report template (#5561)

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

Author: kevmodrome

.github/ISSUE_TEMPLATE/bug_report.md

@@ -7,6 +7,8 @@ assignees: ''
 
 ---
 
+## Is this about svelte@next? This project is currently in a pre-release stage and breaking changes may occur at any time. Please do not post any kind of bug reports or questions on GitHub about it.
+
 **Describe the bug**
 A clear and concise description of what the bug is.