docs: add payment-period conduct note

[silentgeckoaudit3801]

Closes #134.

Summary

• Add a concise payment-period conduct note for AnchorKit GrantFox / Maybe Rewarded contributors.
• Discourage repeated complaints, require self-review before payment follow-up, reference GrantFox evaluation, and point to testing/CI evidence expectations.
• Link the note from README and cross-link it from contributor payment expectations.

Verification

• Docs-only change; no runtime tooling required.
• Compared against upstream main: 3 files changed, 0 commits behind.

[silentgeckoaudit3801]

Thanks for merging this. Is there an official GrantFox reward claim or payment step I should follow for the merged Maybe Rewarded issue #134?