docs: update "How It Works" to reflect default-on assignee behavior

[Copilot]

Pull Request

Description

The "How It Works" section implied users must explicitly enable addPullRequestAuthorAsAssignee, conflicting with the default value of true introduced in #82.

• README.md (line 28): Reworded step 2 from opt-in framing (If ... true is set) to default-on framing (Unless ... false is configured)

Type of change

• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have documented my changes with clear TSDoc comments
• My commit messages follow Conventional Commits / Semantic Release
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• No sensitive information (e.g., secrets, passwords) is included in the code

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[github-actions]

🎉 This PR is included in version 2.1.0 🎉

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