docs: Add comprehensive CONTRIBUTING.md for x402 proxy development

[Kubudak90]

Summary

This PR adds a comprehensive CONTRIBUTING.md file to help onboard developers to x402-proxy's mission of enabling automatic payments for any x402 or MPP endpoint.

What's Included

• Getting Started: Prerequisites, quick start guide, and environment setup including x402/MCP background
• Development Setup: Instructions for local development, testing x402 payments, and chain support
• Project Structure: Clear overview of the codebase organization (CLI, MCP, x402 modules)
• Contributing Workflow: Branch naming conventions, commit message format, and PR process
• Coding Standards: TypeScript guidelines, error handling patterns, and chain configuration
• Testing: Unit tests, integration tests, and manual testing against real endpoints
• Security: Vulnerability reporting process and security best practices for payment handling
• Community: Communication channels and getting help
• Bounty Program: Information about the bounty program with reward tiers
• Resources: Learning materials and tools for x402/MCP development

Why This Matters

As a CLI + MCP proxy enabling automatic payments across Base, Solana, and Tempo, clear contribution guidelines are essential for:

• Attracting developers interested in agentic commerce
• Maintaining security for payment processing
• Ensuring multi-chain compatibility
• Building a community around x402 and MPP standards

The guidelines are specifically tailored for:

• x402 protocol implementation
• MCP (Model Context Protocol) integration
• Multi-chain payment support (Base, Solana, Tempo)
• CLI tool development
• Zero-crypto-code buyer experience

Checklist

• Follows conventional commits style
• Includes x402/MCP-specific development guidance
• Provides clear examples for TypeScript
• Addresses security considerations for payment protocols
• Documents multi-chain testing requirements
• Includes bounty program information

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Happy to address any feedback! Let's build the future of agentic commerce! 🚀