Update contributors guide to explain the default settings for PR branch created from a fork

[idorobotics]

Checklist

• I've read the contribution guidelines.
• I've searched other issues and no duplicate issues were found.
• I've agreed with the maintainers that I can plan this task.

Description

When a PR branch is created from a fork, Github gives the option to Allow edits and access to secrets by maintainers. This option is selected by default.

If a PR author deselects this option, merging can become a challenge since maintainers cannot make edits related to the fork. See, for example, autowarefoundation/autoware.universe#5748 and autowarefoundation/autoware.universe#5731

The contributors guidelines should explain why it is preferred to enable repository maintainer permissions.

Purpose

To streamline merging of PR branches created from forks.

Possible approaches

Update the contribution guideline to explain this.

Definition of done

Documentation updated.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity.