Add unified CONTRIBUTING.md draft (Issue #186)

CONTRIBUTING.md

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+# Contributing Guide
+
+Welcome! We’re glad you’re interested in contributing to projects like ScanCode.io, VulnerableCode, and DejaCode.  
+This guide covers the basics to help you get started quickly and smoothly.
+
+---
+
+## 1. Getting Started
+
+1. **Find or open an issue**  
+   Check the issue tracker for existing discussions.  
+   If you plan a larger change or new feature, please open an issue first so we can align on the approach.
+
+2. **Fork and clone the repository**  
+   Fork the repo on GitHub and clone your fork locally.
+
+3. **Create a new branch**  
+   Use a short, descriptive branch name such as:  
+   - `fix/update-readme`  
+   - `feature/new-api-endpoint`
+
+4. **Develop and test locally**  
+   Write your code and make sure tests pass before opening a Pull Request.
+
+---
+
+## 2. Code Quality and Standards
+
+### 2.1. Code Style
+
+Most AboutCode repositories use **PEP8** conventions. Tools like **Black** and **isort** are commonly used for formatting and import sorting.
+
+To format your code automatically, many projects support:
+
+```bash
+make valid
+```
+
+### 2.2. pre-commit Hooks (Optional but Recommended)
+
+If the repository includes a `.pre-commit-config.yaml`, you can enable automatic checks to run before every commit:
+
+```bash
+pip install pre-commit
+pre-commit install
+```
+
+This helps catch formatting and simple issues early.
+
+### 2.3. Developer Certificate of Origin (DCO)
+
+All contributions require a DCO sign-off.  
+Add this automatically by committing with:
+
+```bash
+git commit -s
+```
+
+Each commit message must include a line like:
+
+```
+Signed-off-by: Your Name <email@example.com>
+```
+
+### 2.4. Tests
+
+If you fix a bug, add a test that fails before your change and passes afterward.  
+If you introduce a new feature, include tests that cover the new functionality.
+
+Run the full test suite locally:
+
+```bash
+make test
+```
+
+---
+
+## 3. Submitting Your Pull Request
+
+When your branch is ready:
+
+1. Push it to your fork.
+2. Open a Pull Request against the main branch of the original repository.
+
+A helpful PR includes:
+
+- [ ] **Linked Issue** (e.g., “Fixes #123”)  
+- [ ] **Passing Tests** (local + CI)  
+- [ ] **DCO Signed** on all commits  
+- [ ] **Changelog/Docs Updated** if the change affects behavior  
+
+Thanks for contributing! We appreciate your time and effort.