docs: add contributing guide

CONTRIBUTING.md

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+# Contributing to uproot
+
+Thank you for your interest in contributing to **uproot**! uproot is a community-driven project, and we welcome contributions of all kinds, including bug reports, feature requests, documentation improvements, and code contributions.
+
+This guide will help you get started with contributing.
+
+---
+
+## 🚀 Quick Start
+
+1. **Fork** the repository on GitHub.
+2. **Clone** your fork locally:
+   ```bash
+   git clone [email protected]:YOUR_USERNAME/uproot5.git
+   cd uproot
+   ```
+
+3. **Set up the development environment**:
+
+   We recommend using a conda environment. You can reproduce the full developer environment with:
+
+   ```bash
+   conda create -n uproot-py313 python=3.13
+   conda activate uproot-py313
+
+   # Add conda-forge channel and prioritize it
+   conda config --add channels conda-forge
+   conda config --set channel_priority strict
+
+   # Install dependencies
+   conda install xrootd
+   conda install root
+   conda install pandas
+   conda install dask
+   conda install minio
+
+   # pip-only dependencies
+   pip install scikit-hep-testdata
+   pip install rangehttpserver
+   pip install boost_histogram
+   pip install hist
+   pip install dask_awkward
+   pip install awkward-pandas
+   pip install pytest-timeout
+   pip install fsspec-xrootd
+
+   # Run local HTTP server (if needed for test data)
+   python -m RangeHTTPServer
+   ```
+
+4. **Install Uproot in editable mode**:
+   ```bash
+   pip install -e ."[dev,test]"
+   ```
+
+---
+
+## 🌿 Branching and Naming Conventions
+
+- Always work on a **feature branch**:
+  ```bash
+  git checkout -b YOUR_USERNAME/my-cool-feature
+  ```
+- Use descriptive names, such as:
+  - `fix_streamer-parsing`
+  - `feature_custom-interpretation-api`
+  - `docs_improve-tutorials`
+
+---
+
+## 🎨 Code Style
+
+- Use **pre-commit** for automatic linting, style issues, and formatting:
+  ```bash
+  pre-commit run --all
+
+---
+
+## ✅ Running Tests
+
+Uproot uses **pytest** for testing.
+
+To run the full test suite:
+```bash
+python -m pytest tests -ss -vv
+```
+
+To run a specific test module:
+```bash
+pytest tests/test_my_module.py
+```
+
+Some tests may depend on having ROOT or XRootD installed. These are covered in the environment setup.
+
+---
+
+## 🔃 Submitting a Pull Request
+
+1. Make sure all tests pass and your code is cleanly formatted.
+2. Push your changes to your fork:
+   ```bash
+   git push origin YOUR_USERNAME/my-cool-feature
+   ```
+3. Open a pull request (PR) from your fork to the `main` branch of [scikit-hep/uproot](https://github.com/scikit-hep/uproot).
+4. Fill in the PR template and explain what you did and why.
+5. Be responsive to feedback from reviewers—we’re all here to help!
+
+---
+
+## 🐛 Reporting Bugs
+
+1. Check if the bug is already reported on the [issue tracker](https://github.com/scikit-hep/uproot/issues).
+2. If not, [open a new issue](https://github.com/scikit-hep/uproot/issues/new/choose) and provide:
+   - A minimal reproducible example
+   - Expected vs. actual behavior
+   - Version info (`python -m uproot --version`)
+   - Any relevant stack trace or logs
+
+---
+
+## 💡 Requesting Features
+
+1. Search the issues to see if a similar feature has been discussed.
+2. If not, open a **Feature Request** issue and describe:
+   - What problem the feature solves
+   - A suggested implementation or interface (if applicable)
+   - Any related prior art in other libraries or experiments
+
+---
+
+## 📚 Improving Documentation
+
+Documentation lives in the `docs-sphinx/` folder and is built using **Sphinx**. To build locally
+make sure you have Sphinx and the ReadTheDocs theme installed in your virtualenv:
+```bash
+pip install sphinx sphinx-rtd-theme
+```
+Navigate to your docs folder and invoke the Sphinx builder to produce HTML in the _build/html directory:
+```
+cd docs-sphinx
+sphinx-build -b html . _build/html
+```
+Once it finishes, open:
+```
+open _build/html/index.html
+```
+
+You can also suggest improvements to examples, tutorials, and API references.
+
+---
+
+## 🙌 Thanks!
+
+uproot thrives on its community. Whether you're fixing a typo, contributing a feature, or suggesting a design—you're making a difference!