Document the complete reward flow in README

[AbdulSnk]

Overview

The streamforge-stellar-rewards repository lacks clear documentation explaining how the reward flow works end-to-end. Contributors and maintainers need to understand how USDC rewards move from escrow to a contributor's Stellar wallet before they can safely work on this codebase.

What needs to be done

Update the existing README.md to include the following sections:

• Overview — what this repo does in plain language
• How the reward flow works — step by step explanation of:
  • How escrow is created and funded
  • What conditions trigger a release
  • How USDC is sent to the contributor's Stellar wallet
  • What happens when a release fails
• Setup instructions — how to install dependencies and run the project locally
• Environment variables — list all required .env variables with descriptions
• Running tests — how to run the test suite
• Contributing — link to or include basic contribution guidelines

Acceptance Criteria

• All sections listed above are present in README.md
• Reward flow explanation is clear enough for a first-time contributor
• Setup instructions work correctly when followed from scratch
• All environment variables are documented
• README is free of spelling and grammar errors

Notes

Write for a developer who has never seen this codebase before. Avoid jargon where possible. A simple diagram of the reward flow is a bonus but not required.