docs: add payment-period communication policy (#500)

[Terryodib]

Add a professional, firm conduct policy for contributor communication during the payment evaluation period.

• New docs/payment-period-communication-policy.md discouraging spam and pressure, requiring self-review before raising payment concerns, and referencing the GrantFox evaluation process.
• Link the policy from the README documentation section.
• Add a Payment-Period Communication subsection to CONTRIBUTING.md.

Closes #500

Summary

Closes #

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Acceptance Criteria

Acceptance criterion	Implementation or evidence	Status
		Pass / Not applicable

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