Add a CONTRIBUTING.md file with checks to execute before creating a PR

CONTRIBUTING.md

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+# Contributing
+
+When contributing to this repository, please first discuss the change you wish to make via issue with the owners of this repository before making a change.
+
+## Pull Request Process
+
+1. Ensure any install or build dependencies are removed before the end of the layer when doing a
+   build.
+2. Update the README.md with details of changes to the interface, this includes new environment
+   variables, exposed ports, useful file locations and container parameters.
+3. Update the `CHANGELOG.md` file with your changes under the `[Unreleased]` section.
+
+## Development Setup
+
+1.  Install `pipenv`.
+2.  Install dependencies with `pipenv install --dev`.
+
+## Running Checks
+
+Before submitting a pull request, please run the following checks to ensure your code is clean and passes all tests.
+
+### Code Formatting
+
+Run `black` to format your code:
+
+```bash
+pipenv run black .
+```
+
+### Linting
+
+Run `pylint` to check for code style issues:
+
+```bash
+pipenv run pylint systemctl_mqtt
+pipenv run pylint tests/*
+```
+
+### Static Type Checking
+
+Run `mypy` to check for type errors:
+
+```bash
+pipenv run mypy systemctl_mqtt tests
+```
+
+### Testing
+
+Run `pytest` to run the test suite:
+
+```bash
+pipenv run pytest --cov=systemctl_mqtt --cov-report=term-missing --cov-fail-under=100
+```