chore(ci): add note about cronjob

Author: heitorlessa

.github/workflows/on_schedule_monthly_roadmap_reminder.yml

@@ -3,7 +3,7 @@ name: Monthly roadmap reminder
 on:
     workflow_dispatch: {}
     schedule:
-      - cron: '0 0 1 * *'
+      - cron: '0 0 1 * *'  # runs first day of the month
 
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     contents: read