Update contributing guide.

Author: fkdosilovic

docs/source/contributors_guide/contributing.rst

@@ -25,4 +25,46 @@ Preparing the development setup
 .. code-block:: console
 
     $ pip install -e .[test]
+    $ pip install -r docs/requirements.txt # needed for building the docs
     $ pre-commit install
+
+Building documentation
+----------------------
+
+Documentation is built using Sphinx. To build the documentation, run the
+
+.. code-block:: console
+
+    $ cd docs
+    $ make html
+
+You should inspect the changes in the documentation by opening the
+``docs/build/html/index.html`` file in your browser.
+
+You'll see a different output since the documentation is build with
+`sphinx-multiversion <https://github.com/sphinx-contrib/multiversion>`_ extension.
+
+Testing
+-------
+
+If you implemented a feature or fixed a bug, please add tests for it.
+
+Unfortunately, at the moment you cannot run full test suite because it requires
+access to API keys for all implemented API hubs (ATD, Sulu, and RapidAPI) and
+a private Judge0 instance. To partially address this situation, you can run and
+test your implemented feature and tests locally and use the GitHub CI pipeline
+to run the full test suite.
+
+To run the tests locally, you can use the following command:
+
+.. code-block:: console
+
+    $ pytest -svv tests -k '<test_name>'
+
+To make the test compatible with the CI pipeline, you should use one of the
+client fixtures:
+
+.. code-block:: python
+    
+    def test_my_test(request):
+        client = request.getfixturevalue("judge0_ce_client") # or judge0_extra_ce_client