Add CONTRIBUTING.md for community contribution guide

Author: FMATheNomad

CONTRIBUTING.md

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+# 🤝 Contributing to aiverify
+
+First off, thank you for considering contributing to aiverify! Every star, issue, and pull request helps make AI-generated code safer for everyone.
+
+## ⭐ Support the Project
+
+- **Star the repo** — it helps others discover aiverify
+- **Sponsor** — [GitHub Sponsors](https://github.com/sponsors/FMATheNomad) supports a solo founder
+- **Share** — tell your team about aiverify on [Twitter](https://x.com/intent/tweet?text=Verify%20AI-generated%20code%20before%20you%20commit%E2%80%94aiverify%20catches%20hallucinations%20in%20Claude%2C%20Copilot%2C%20Cursor%2C%20ChatGPT%20code.&url=https://github.com/FMATheNomad/aiverify)
+
+## 🧠 Adding a New Rule
+
+1. **Create the rule class** in `aiverify/rules/` extending `BaseRule`:
+
+```python
+from .base import BaseRule
+
+class MyNewRule(BaseRule):
+    name = "my-new-rule"
+    code = "PY006"  # or JS006, GEN006
+    description = "What this rule detects"
+    severity = "warning"  # error | warning | info
+    language = "python"   # python | javascript | generic
+
+    def check(self, tree, source: bytes) -> list[Finding]:
+        # Use tree-sitter queries here (NOT regex)
+        # Return list of Finding objects
+        pass
+```
+
+2. **Register the rule** — add it to the `*_RULES` list in the appropriate module (`python_rules.py`, `js_rules.py`, or `generic.py`)
+
+3. **Add tests** in `tests/test_rules.py`:
+   - Test that the rule detects the bad pattern
+   - Test that clean code doesn't trigger false positives
+
+4. **Run tests**:
+```bash
+python -m pytest tests/ -v
+```
+
+### Rule Guidelines
+
+- ✅ **Must use tree-sitter queries** — no regex-based AST analysis
+- ✅ **Must have tests** — both positive (detects pattern) and negative (no false positive)
+- ✅ **Must handle edge cases** — empty files, syntax errors, nested structures
+- ❌ **No false positives** — better to miss a pattern than to flag clean code
+- ❌ **No external dependencies** — keep it self-contained
+
+## 🐛 Reporting Issues
+
+Open an issue at https://github.com/FMATheNomad/aiverify/issues with:
+- The command you ran
+- The output (or error message)
+- A minimal example that reproduces the issue
+- Your Python version and OS
+
+## 📦 Development Setup
+
+```bash
+git clone https://github.com/FMATheNomad/aiverify.git
+cd aiverify
+python -m venv venv
+source venv/bin/activate
+pip install -r requirements.txt
+pip install -e .
+python -m pytest tests/ -v
+```
+
+## 📜 Code of Conduct
+
+Be respectful. Be constructive. Be excellent to each other. This is a solo founder project running on kindness and coffee.
+
+---
+
+*Built by a solo founder. Every star & sponsor helps keep this project alive.* 🙏