Enchance README

[andr-drgm]

Description

[notjamyfriench]

Submitted a fresh README rewrite in PR #44, aligned with the current USDC bounty workflow and webhook/API setup. Also submitted via Collaborators Submit Solution for bounty tracking.

[nicovaleops]

Submitted a docs-only PR for this bounty: #49

What changed:

• rewrote the README around the bounty marketplace workflow
• added clearer contributor/poster setup steps
• documented env vars, local development, deployment, troubleshooting, and security notes
• kept the patch README-only for a low-risk review

Validation: reviewed against the current repository structure and kept the copy focused on the actual Collaborators product/workflow.