Add CONTRIBUTING.md for KFP pipeline development

CONTRIBUTING.md

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+# Contributing to Data Processing
+
+Thank you for your interest in contributing. This guide covers KFP pipeline development.
+
+## Prerequisites
+
+- Python 3.12
+- Git
+- [pre-commit](https://pre-commit.com/) (`pip install pre-commit`)
+- Docker (only for local pipeline testing via `local_run.py`)
+
+## Development Setup
+
+1. **Fork and clone** the repository:
+
+   ```bash
+   git clone https://github.com/<your-fork>/data-processing.git
+   cd data-processing
+   ```
+
+2. **Create a virtual environment**:
+
+   ```bash
+   python3.12 -m venv .venv
+   source .venv/bin/activate
+   ```
+
+3. **Install dev dependencies**:
+
+   ```bash
+   pip install -r requirements-dev.txt
+   ```
+
+   For pipeline work, also install pipeline-specific dependencies:
+
+   ```bash
+   pip install -r kubeflow-pipelines/docling-standard/requirements.txt
+   # or
+   pip install -r kubeflow-pipelines/docling-vlm/requirements.txt
+   ```
+
+4. **Install pre-commit hooks**:
+
+   ```bash
+   pre-commit install
+   ```
+
+## Building and Running Tests
+
+```bash
+make help              # Show all available targets
+make lint              # Ruff lint + format check
+make format-python     # Auto-format Python files
+```
+
+Run directly with pytest:
+
+```bash
+pytest tests/unit/ -v                    # Unit tests only
+pytest tests/unit/test_kfp_constants.py  # Specific test file
+```
+
+## Submitting a Pull Request
+
+1. **Open an issue first** describing what you want to change.
+
+2. **Create a feature branch**:
+
+   ```bash
+   git checkout -b feature/your-change
+   ```
+
+3. **Make your changes** and verify locally:
+
+   ```bash
+   make lint              # Must pass
+   ```
+
+4. **For pipeline or component changes**, recompile the YAML and commit it:
+
+   ```bash
+   cd kubeflow-pipelines/docling-standard && python standard_convert_pipeline.py
+   # or
+   cd kubeflow-pipelines/docling-vlm && python vlm_convert_pipeline.py
+   ```
+
+   CI will fail if compiled YAML doesn't match the committed version.
+
+5. **Push and open a PR**. CI runs automatically:
+
+   | Check | What it validates |
+   |---|---|
+   | `validate-python.yml` | Ruff lint + format check |
+   | `compile-kfp.yml` | Pipeline YAML matches compiled output |
+
+6. **Get one approval** from `@opendatahub-io/odh-data-processing`. PRs are squash-merged via Mergify.
+
+## KFP Component Guidelines
+
+KFP `@dsl.component` functions are serialized into isolated containers:
+
+- **Imports must be inside the function body**, not at the top of the file
+- Shared modules (`common/components.py`) only work at compile time
+- You'll see `# pylint: disable=import-outside-toplevel` — this is intentional
+
+## Generated Files
+
+Files ending in `_compiled.yaml` are auto-generated. Do not edit them directly — edit the source `.py` file and regenerate.
+
+## Adding a New KFP Component
+
+1. Add the component function to `kubeflow-pipelines/common/components.py` (if shared) or the pipeline-specific `*_components.py` file
+2. Use `@dsl.component` with inline imports
+3. Wire it into the pipeline definition (`*_convert_pipeline.py`)
+4. Recompile: `python *_convert_pipeline.py`
+5. Commit both the source `.py` and the compiled `_compiled.yaml`
+
+## Getting Help
+
+- Run `make help` for all available commands
+- See `CLAUDE.md` for architecture overview and debugging
+- See `docs/maintainers/release-strategy.md` for release process
+- Open an issue for questions or problems