Add a guide for contributors

Author: LBeaudoux

CONTRIBUTING.md

@@ -0,0 +1,112 @@
+# Contributing
+
+
+Thanks for taking the time to contribute to `iso639-lang`!
+
+If you want to add or improve a feature, please create a GitHub issue to discuss your proposal before you submit a pull request.
+
+
+## Pull Request Guidelines
+
+- Keep your pull request focused on a single feature or change.
+- Include tests for any new features or changes.
+- Update the documentation if necessary.
+- Make sure all tests pass.
+- Ensure that your code follows the project's code style.
+
+
+## Creating a Development Environment
+
+Fork the repository.
+
+```
+https://github.com/LBeaudoux/iso639/fork
+```
+
+Clone the repository.
+
+```sh
+git clone https://github.com/your-username/iso639.git
+cd iso639
+```
+
+Create a virtual environment.
+
+```sh
+python3 -m venv env_iso639
+source env_iso639/bin/activate
+```
+
+Build and install `iso639-lang` in 'editable' mode.
+
+```sh
+pip install -e .
+pip install -r requirements-dev.txt
+```
+
+
+## Running Tests
+
+```sh
+pytest tests dev_tests
+```
+
+The main tests are located in `tests/`. They run faster and don't require access to the original ISO 639 data files located in `dev_iso639/downloads/`.
+
+The tests in `dev_tests` are slower because they check that the `iso639-lang` library is fully consistent with the official ISO 639 tables.
+
+Pushing changes to your forked repository should trigger a [workflow](https://github.com/LBeaudoux/iso639/actions/workflows/ci.yml) that tests your code across multiple operating systems and Python versions.
+
+
+## Running Code Style Checks
+
+```sh
+flake8
+black --check .
+isort --check-only .
+```
+
+`iso639-lang` uses `flake8` for linting, `black` for formatting and `isort` for sorting imports. The configuration of these tools is available in the `.flake8` and `pyproject.toml` files.
+
+
+## Submitting Changes
+
+Specify a new remote upstream repository to sync with the forked repository.
+
+```sh
+git remote add upstream https://github.com/LBeaudoux/iso639.git
+```
+
+Check that your local repository has a remote link to both your forked repository (origin) and `LBeaudoux/iso639` (upstream).
+
+```sh
+git remote -v
+```
+
+Make sure your local master branch is up to date.
+
+```sh
+git checkout master
+git pull upstream master
+``` 
+
+Create a new branch for your changes.
+   
+```sh
+git checkout -b new-branch-name
+```
+
+Commit your changes with a clear and concise commit message.
+
+```sh
+git commit -m "Add a brief description of your changes"
+```
+
+Push your changes to your forked repository.
+
+```sh
+git push origin new-branch-name
+```
+
+Navigate to your forked repository and click on the 'New Pull Request' button. Select your branch and provide a description of your changes.
+