docs: add payment-period communication policy

[DevALVIN-24]

Closes #147

Summary

This PR adds a firm and professional payment-period communication policy for contributors to discourage spam and repeated complaints regarding reward distribution.

Implementation details

• CONTRIBUTING.md: Added a new "Conduct & Payment Policy" section directly after the initial contribution guidelines, making it highly visible to contributors starting their work.
• The section enforces professional conduct, mandates rigorous self-review before making payment inquiries, and clearly outlines the GrantFox evaluation process.
• The section links to the detailed docs/payment-period-conduct.md file (which is already linked in the README.md).

Acceptance Criteria Covered

• Payment-period communication policy is added.
• Repeated spam or complaints are discouraged.
• Contributors are asked to self-review before raising payment concerns.
• GrantFox evaluation process is referenced.
• Tone is professional and firm.
• README links to the policy (pre-existing coverage verified).