Add contract local development guide

[Brainoncrack]

Description

Difficulty: Intermediate

Type: Documentation

Summary

Create a local development guide for the contracts repo.

Current Behaviour

New contributors may struggle to build, test, and understand Soroban contract workflows.

Expected Behaviour

The repository should explain prerequisites, commands, test structure, and common issues.

Suggested Implementation

Add a local development document and link it from the README.

Files or Areas Likely Affected

docs/
README.md
Cargo.toml
tests/

Acceptance Criteria

• Local development guide is added.
• Prerequisites are listed.
• Build and test commands are included.
• Common issues are covered.
• README links to the guide.

Additional Notes

This improves contributor onboarding.