Improve contents of CONTRIBUTING.md listing all tox environments

Author: althonos

CONTRIBUTING.md

@@ -2,14 +2,122 @@
 
 Pull Requests are very welcome for this project!
 
-For bug fixes or new features, please file an issue before submitting a pull request. If the change isn't trivial, it may be best to wait for feedback. For a quicker response, contact [Will McGugan](mailto:[email protected]) directly.
+For bug fixes or new features, please file an issue before submitting a pull
+request. If the change isn't trivial, it may be best to wait for feedback.
+For a quicker response, contact [Will McGugan](mailto:[email protected])
+directly.
 
-## Coding Guidelines
 
-This project runs on Python2.7 and Python3.X. Python2.7 will be dropped at some point, but for now, please maintain compatibility.
+## `tox`
+
+Most of the guidelines that follow can be checked with a particular
+[`tox`](https://pypi.org/project/tox/) environment. Having it installed will
+help you develop and verify your code locally without having to wait for
+our Continuous Integration pipeline to finish.
 
-Please format new code with [black](https://github.com/ambv/black), using the default settings.
 
 ## Tests
 
-New code should have unit tests. We strive to have near 100% coverage. Get in touch, if you need assistance with the tests.
+New code should have unit tests. We strive to have near 100% coverage.
+Get in touch, if you need assistance with the tests. You shouldn't refrain
+from opening a Pull Request even if all the tests were not added yet, or if
+not all of them are passing yet.
+
+### Dependencies
+
+The dependency for running the tests can be found in the `tests/requirements.txt` file.
+If you're using `tox`, you won't have to install them manually. Otherwise,
+they can be installed with `pip`:
+```console
+$ pip install -r tests/requirements.txt
+```
+
+### Running (with `tox`)
+
+Simply run in the repository folder to execute the tests for all available
+environments:
+```console
+$ tox
+```
+
+Since this can take some time, you can use a single environment to run
+tests only once, for instance to run tests only with Python 3.9:
+```console
+$ tox -e py39
+```
+
+### Running (without `tox`)
+
+Tests are written using the standard [`unittest`](https://docs.python.org/3/library/unittest.html)
+framework. You should be able to run them using the standard library runner:
+```console
+$ python -m unittest discover -vv
+```
+
+
+## Coding Guidelines
+
+This project runs on Python2.7 and Python3.X. Python2.7 will be dropped at
+some point, but for now, please maintain compatibility. PyFilesystem2 uses
+the [`six`](https://pypi.org/project/six/) library to write version-agnostic
+Python code.
+
+### Style
+
+The code (including the tests) should follow PEP8. You can check for the
+code style with:
+```console
+$ tox -e codestyle
+```
+
+This will invoke [`flake8`](https://pypi.org/project/flake8/) with some common
+plugins such as [`flake8-comprehensions`](https://pypi.org/project/flake8-comprehensions/).
+
+### Format
+
+Please format new code with [black](https://github.com/ambv/black), using the
+default settings. You can check whether the code is well-formatted with:
+```console
+$ tox -e codeformat
+```
+
+### Type annotations
+
+The code is typechecked with [`mypy`](https://pypi.org/project/mypy/), and
+type annotations written as comments, to stay compatible with Python2. Run
+the typechecking with:
+```console
+$ tox -e typecheck
+```
+
+
+## Documentation
+
+### Dependencies
+
+The documentation is built with [Sphinx](https://pypi.org/project/Sphinx/),
+using the [ReadTheDocs](https://pypi.org/project/sphinx-rtd-theme/) theme.
+The dependencies are listed in `docs/requirements.txt` and can be installed with
+`pip`:
+```console
+$ pip install -r docs/requirements.txt
+```
+
+### Building
+
+Run the following command to build the HTML documentation:
+```console
+$ python setup.py build_sphinx
+```
+
+The documentation index will be written to the `build/sphinx/html/`
+directory.
+
+### Style
+
+The API reference is written in the Python source, using docstrings in
+[Google format](https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html).
+The documentation style can be checked with:
+```console
+$ tox -e docstyle
+```