Update documentation: Missing Guideline in the Burndown Chart.
https://confluence.atlassian.com/jirakb/missing-guideline-in-the-burndown-chart-779158658.html

Include the following situation.

Issues where the release date is earlier than the start of the sprint will show as "Issue added to sprint after start time" and due to this, the guideline won't show up if all issues are on the same condition.