docs: add CONTRIBUTING.md with branching strategy

CONTRIBUTING.md

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+# Contributing to FinEE
+
+Thank you for your interest in contributing! Here's how to get started.
+
+## 🌿 Branching Strategy
+
+We use **Git Flow** for professional development:
+
+```
+main          ────●────●────●────────────●─────→  (stable releases only)
+                  │    │    │            │
+                  │    │    └──tag:v1.0.3│
+                  │    │                 │
+develop       ────●────●────●────●───────●─────→  (integration branch)
+                       │    │    │
+feature/xyz   ─────────●────●────┘               (feature branches)
+```
+
+### Branches
+
+| Branch | Purpose | Merge To |
+|--------|---------|----------|
+| `main` | Stable releases only | - |
+| `develop` | Integration & testing | `main` (via PR) |
+| `feature/*` | New features | `develop` (via PR) |
+| `fix/*` | Bug fixes | `develop` (via PR) |
+| `hotfix/*` | Urgent production fixes | `main` + `develop` |
+
+### Workflow
+
+1. **New Feature**:
+   ```bash
+   git checkout develop
+   git pull origin develop
+   git checkout -b feature/my-feature
+   # ... make changes ...
+   git push -u origin feature/my-feature
+   # Create PR to develop
+   ```
+
+2. **Bug Fix**:
+   ```bash
+   git checkout develop
+   git checkout -b fix/issue-123
+   # ... fix bug ...
+   git push -u origin fix/issue-123
+   # Create PR to develop
+   ```
+
+3. **Release**:
+   ```bash
+   git checkout main
+   git merge develop
+   git tag -a v1.x.x -m "Release v1.x.x"
+   git push origin main --tags
+   ```
+
+## 🧪 Running Tests
+
+```bash
+# Install dev dependencies
+pip install -e ".[dev]"
+
+# Run all tests
+pytest tests/ -v
+
+# Run specific test
+pytest tests/test_regex_engine.py -v
+
+# Run with coverage
+pytest tests/ --cov=finee --cov-report=html
+```
+
+## 📝 Code Style
+
+- **Black** for formatting (line length: 100)
+- **Ruff** for linting
+- **Type hints** for all public functions
+
+```bash
+# Format code
+black src/ tests/
+
+# Lint
+ruff check src/ tests/
+
+# Type check
+mypy src/finee/
+```
+
+## 🚀 Publishing
+
+Only maintainers can publish to PyPI:
+
+```bash
+# Bump version in pyproject.toml
+# Build
+python -m build
+
+# Upload
+twine upload dist/*
+```
+
+## 📋 Commit Messages
+
+Use conventional commits:
+
+```
+feat: add support for Lakhs notation
+fix: handle Unicode ₹ symbol in regex
+docs: update README with torture tests
+test: add edge case tests for truncated SMS
+chore: move notebooks to experiments/
+```
+
+## 🙏 Thank You!
+
+Every contribution helps make FinEE better for the Indian fintech community.