Update contributing guide

Author: fnesveda

CONTRIBUTING.md

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+# Development
+
+## Environment
+
+For local development, it is required to have Python 3.8 installed.
+
+It is recommended to set up a virtual environment while developing this package to isolate your development environment,
+however, due to the many varied ways Python can be installed and virtual environments can be set up,
+this is left up to the developers to do themselves.
+
+One recommended way is with the built-in `venv` module:
+
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+```
+
+To improve on the experience, you can use [pyenv](https://github.com/pyenv/pyenv) to have an environment with a pinned Python version,
+and [direnv](https://github.com/direnv/direnv) to automatically activate/deactivate the environment when you enter/exit the project folder.
+
+## Dependencies
+
+To install this package and its development dependencies, run `make install-dev`
+
+## Formatting
+
+We use `autopep8` and `isort` to automatically format the code to a common format. To run the formatting, just run `make format`.
+
+## Linting, type-checking and unit testing
+
+We use `flake8` for linting, `mypy` for type checking and `pytest` for unit testing. To run these tools, just run `make check-code`.
+
+## Integration tests
+
+We have integration tests which build and run actors using the Python SDK on the Apify Platform.
+To run these tests, you need to set the `APIFY_TEST_USER_API_TOKEN` environment variable to the API token of the Apify user you want to use for the tests,
+and then start them with `make integration-tests`.
+
+If you want to run the integration tests on a different environment than the main Apify Platform,
+you need to set the `APIFY_INTEGRATION_TESTS_API_URL` environment variable to the right URL to the Apify API you want to use.
+
+## Documentation
+
+We use the [Google docstring format](https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html) for documenting the code.
+We document every user-facing class or method, and enforce that using the flake8-docstrings library.
+
+The documentation is then rendered from the docstrings in the code, using `pydoc-markdown` and some heavy post-processing,
+and from Markdown documents in the `docs` folder in the `docs` branch, and then rendered using Docusaurus and published to GitHub pages.
+
+## Release process
+
+Publishing new versions to [PyPI](https://pypi.org/project/apify) happens automatically through GitHub Actions.
+
+On each commit to the `master` branch, a new beta release is published, taking the version number from `src/apify/_version.py`
+and automatically incrementing the beta version suffix by 1 from the last beta release published to PyPI.
+
+A stable version is published when a new release is created using GitHub Releases, again taking the version number from `src/apify/_version.py`.
+The built package assets are automatically uploaded to the GitHub release.
+
+If there is already a stable version with the same version number as in `src/apify/_version.py` published to PyPI, the publish process fails,
+so don't forget to update the version number before releasing a new version.
+The release process also fails when the released version is not described in `CHANGELOG.md`,
+so don't forget to describe the changes in the new version there.