Add CONTRIBUTING.md for OpenCare-Core contributors

## 📌 Problem
There is no structured guide for contributors.

## 🎯 Objective
Create a contribution guideline for developers.

## ✅ Tasks
- Add `CONTRIBUTING.md`
- Include:
  - how to fork and clone
  - branching strategy
  - commit message style
  - PR process

## 🧪 Acceptance Criteria
- File exists in repo root
- README links to it
- New contributors can follow it easily

