docs: add CONTRIBUTING.md for local development workflow

CONTRIBUTING.md

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+# Contributing to ollama-python
+
+Thanks for your interest in contributing! This guide covers the local development workflow used by this repository.
+
+## Prerequisites
+
+- Python 3.8+
+- [uv](https://docs.astral.sh/uv/) (recommended) or another way to run Hatch environments
+- [Ollama](https://ollama.com/download) installed if you want to exercise live examples against a local server
+
+## Setup
+
+Clone the repository and install the package in editable mode with development tools via Hatch:
+
+```sh
+git clone https://github.com/ollama/ollama-python.git
+cd ollama-python
+uvx hatch env create
+```
+
+Alternatively, with pip:
+
+```sh
+pip install -e .
+pip install pytest pytest-anyio pytest-httpserver ruff
+```
+
+## Repository layout
+
+- `ollama/` — library source (`_client.py`, `_types.py`, `_utils.py`)
+- `tests/` — unit tests (HTTP server fixtures; no live Ollama required for CI)
+- `examples/` — usage examples
+- `pyproject.toml` — project metadata, Ruff, and Hatch configuration
+
+## Tests
+
+Run the full test suite the same way CI does:
+
+```sh
+uvx hatch test -acp
+```
+
+Or with pytest directly after installing test dependencies:
+
+```sh
+pytest ollama tests
+```
+
+## Linting and formatting
+
+This project uses [Ruff](https://docs.astral.sh/ruff/) with settings from `pyproject.toml` (2-space indent, single quotes).
+
+Check formatting and lint (CI commands):
+
+```sh
+uvx hatch fmt --check -f
+uvx hatch fmt --check -l
+```
+
+Apply fixes locally:
+
+```sh
+uvx hatch fmt
+```
+
+Also keep lockfiles in sync when changing dependencies:
+
+```sh
+uv lock --check
+uv export > requirements.txt
+```
+
+## Pull requests
+
+- Create a branch from `main` and keep the change focused
+- Ensure tests and Ruff checks pass before opening a PR
+- Prefer clear commit messages that explain *why*
+- Link related issues in the PR description (for example `Fixes #123`)
+- Draft PRs are welcome for early feedback
+
+## Questions
+
+Open an issue at https://github.com/ollama/ollama-python/issues if something in this guide is unclear or outdated.