Create CONTRIBUTING_BACKEND.md Guide

[Yunusabdul38]

Create CONTRIBUTING_BACKEND.md Guide

Description

Write a focused contribution guide for the newly initialized backend to match the contract direction.

Requirements

• Simple implementation with clear documentation.
• Follow Soroban/Rust best practices.
• Ensure CI checks pass.

Task for Contributor

• Implement the requested change.
• Add a unit test verifying the fix.
• Ensure code is well-formatted.

Verification

• Run 'cargo test' and ensure it passes.
• Verify locally with Soroban CLI if applicable.