Add contributing guide

Author: coloursofnoise

CONTRIBUTING.md

@@ -0,0 +1,70 @@
+# CONTRIBUTING
+
+## Asking for help
+If you are looking for help with setting up or using `mons`, or you have other questions about how the program works and how you can contribute, please use the `#modding_help` or `#modding_dev` channels in the Celeste Community Discord: https://discord.gg/celeste
+
+## Before contributing
+- Check the [existing issues](https://github.com/coloursofnoise/mons/issues) to see if your request has already been made.
+  - If so, add a comment to describe how you plan to resolve it and request to have the issue assigned to you.
+- If an issue does not already exist, create one describing your request and how you plan on implementing it.
+
+## First time setup
+- Clone the GitHub repository locally
+```console
+$ git clone https://github.com/coloursofnoise/mons.git
+$ cd mons
+```
+- Create a virtualenv
+```console
+$ python3 -m venv env
+$ . env/bin/activate
+```
+- Upgrade `pip` and `setuptools`
+```console
+$ python -m pip install --upgrade pip setuptools
+```
+- Install development dependencies, then install `mons` in editable mode
+```console
+$ pip install -r requirements-dev.txt
+$ pip install -e .
+```
+- Install the [pre-commit](https://pre-commit.com) hooks
+```console
+$ pre-commit install
+```
+
+## Writing code
+When making changes to the project, follow best practices for code and version control wherever possible:
+- Commit frequently, with clear commit messages.
+- Use [Black](https://black.readthedocs.io) to format your code. This and other tools will run automatically if you install [pre-commit](https://pre-commit.com) using the instructions above.
+- Add tests to verify your code. Added tests should fail without your changes.
+- Add or update docstrings and other relevant documentation.
+- When creating your pull request, provide a short explanation for your changes and link to the issue being addressed: https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue
+
+## Running tests
+### `pytest`
+Run tests for the current environment with `pytest`
+```console
+$ pytest
+```
+To run end-to-end tests using a local Celeste install, run `pytest` and provide the path for the Celeste install
+
+:warning: When tests fail they can break the install they are run on. **Do not use a Celeste install you care about.**
+```console
+$ pytest --mons-test-install={/path/to/Celeste/install}
+```
+### `tox`
+The full suite of tests can be run with `tox`. This will also be run by the CI server when a pull request is submitted
+```console
+$ tox
+```
+End-to-end tests can also be run with `tox`
+```console
+$ tox -- --mons-test-install={/path/to/Celeste/install}
+```
+
+# MAINTAINERS
+## Releasing a new version
+- Tag a commit with the format `v{version number}` where {version number} follows the [semver](https://semver.org/) specification.
+- The CI server will run tests, then build and upload the new version to [PyPI](https://pypi.org/project/mons/).
+- (Optional) Create a [GitHub release](https://github.com/coloursofnoise/mons/releases) for the new tag once CI has completed successfully.