Add CONTRIBUTING.md to guide new contributors

[kmuwanga83]

📌 Problem

There is no clear contribution guideline for developers, making onboarding difficult.

🎯 Objective

Create a CONTRIBUTING.md file with standard contribution practices.

✅ Tasks

• Add guidelines for:
  • Forking the repository
  • Creating branches
  • Commit message style
  • Pull request process
• Include coding standards (Java + frontend if applicable)
• Add issue reporting guidelines

🧪 Acceptance Criteria

• CONTRIBUTING.md exists in root directory
• README links to it
• New contributors can follow it without confusion

💡 Suggested Labels

documentation, community, good first issue