docs: add contribution guidlines for extras

Author: begumcig

CONTRIBUTING.md

@@ -112,6 +112,10 @@ For specific test markers:
 uv run pytest -m "cpu and not slow"
 ```
 
+#### Adding tests that need optional extras
+
+If a test needs an optional dependency group, add the extra in `pyproject.toml` under `[project.optional-dependencies]`, register a matching `requires_<extra>` marker in `tests/conftest.py`, and mark the tests with it. Add the extra to the test workflow matrix with its install option and marker filter, for example `extras: "--extra my-extra"` and `mark_filter: "requires_my_extra"`. Tests without optional extras are selected with `no_extras`.
+
 #### If you used Option B (pip/conda):
 
 ```bash

docs/contributions/how_to_contribute.rst

@@ -131,6 +131,12 @@ For specific test markers:
 
     uv run pytest -m "cpu and not slow"
 
+**Adding tests that need optional extras**
+
+If a test needs an optional dependency group, add the extra in ``pyproject.toml`` under ``[project.optional-dependencies]``, register a matching ``requires_<extra>`` marker in ``tests/conftest.py``, and mark the tests with it.
+Add the extra to the test workflow matrix with its install option and marker filter, for example ``extras: "--extra my-extra"`` and ``mark_filter: "requires_my_extra"``.
+Tests without optional extras are selected with ``no_extras``.
+
 **If you used Option B (pip/conda):**
 
 .. code-block:: bash