Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Setting up an environment
+
+Clone the `asyncio-mqtt`.
+
+Inside the repository, create a virtual environment.
+
+```bash
+python3 -m venv env
+```
+
+Activate the virtual environment.
+
+```bash
+source ./env/bin/activate
+```
+
+Upgrade `pip`.
+
+```bash
+pip install --upgrade pip
+```
+
+Install the development dependencies.
+
+```bash
+pip install -e .[tests,lint,format]
+```
+
+Install [pre-commit](https://pre-commit.com/) so that your code is formatted and checked when you are doing a commit.
+
+```bash
+pip install pre-commit
+pre-commit install
+```
+
+### Visual Studio Code
+
+If you are using VSCode, here are the settings to activate on save,
+
+- `black` and `isort` to format.
+- `flake8` and `mypy` to lint.
+
+```json
+{
+  "[python]": {
+    "editor.formatOnSave": true,
+    "editor.codeActionsOnSave": {
+      "source.organizeImports": true
+    }
+  },
+  "python.formatting.provider": "black",
+  "python.linting.flake8Enabled": true,
+  "python.linting.mypyEnabled": true
+}
+```
+
+## Testing
+
+To test the code use [pytest](https://docs.pytest.org/en/7.1.x/).
+
+```bash
+pytest
+```
+
+To do the full coverage of `asyncio-mqtt`, run the following command.
+
+```bash
+pytest --cov=src --cov=tests --cov-report=html
+```
+
+To view the coverage open `htmlcov/index.html`.
+
+## Committing
+
+After doing `git commit`, `pre-commit` will check the committed code.
+The check can be passed, skipped or failed.
+If the check failed, it is possible it auto-fixed the code, so you will only need to stage and commit again for it to pass.
+If it did not auto-fixed the code, you will need to do it manually.
+`pre-commit` will only check the code that is staged, the unstaged code will be stashed during the checks.