Update bug_report.md

Author: ota-meshi

.github/ISSUE_TEMPLATE/bug_report.md

@@ -51,4 +51,8 @@ about: Create a report to help us improve
 -->
 
 **Repository to reproduce this issue**
+<!--
+  Please share a repository that can reproduce your issue.
+  If you don't share it, we will most likely add a comment asking you to share the repository.
+-->