Document how to use dangerbot from forks
What does this MR do?
This explains how to workaround one of the dangerbot limitations.
Author's checklist
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Follow the Documentation Guidelines and Style Guide. 
- [-] If applicable, update the permissions table.
- [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
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Apply the documentation label. 
Review checklist
All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.
1. Primary Reviewer
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Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes. 
2. Technical Writer
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Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage. 
3. Maintainer
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Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review. 
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Ensure a release milestone is set. 
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If there has not been a technical writer review, create an issue for one using the Doc Review template. 
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