docs: add contribution guidlines for extras

CONTRIBUTING.md

@@ -112,6 +112,10 @@ For specific test markers:
 uv run pytest -m "cpu and not slow"
 ```
 
+#### How and when to add a new optional extra
+
+Add a new extra when a feature or algorithm needs dependencies that should not be installed for every Pruna user, for example because they are heavy, platform-specific, or only needed for one backend. Define the dependency group in `pyproject.toml` under `[project.optional-dependencies]`. For algorithms, set `required_install` on the algorithm class, for example `required_install = "uv pip install 'pruna[my-extra]'"`; this is used in the import error shown when the extra is missing. Finally, register a matching `requires_<extra>` marker in `tests/conftest.py`, mark tests that need the extra, and add the extra to the CI matrix with its install option and marker filter, for example `extras: "--extra my-extra"` and `mark_filter: "requires_my_extra"`.
+
 #### If you used Option B (pip/conda):
 
 ```bash

docs/contributions/how_to_contribute.rst

@@ -131,6 +131,13 @@ For specific test markers:
 
     uv run pytest -m "cpu and not slow"
 
+**How and when to add a new optional extra**
+
+Add a new extra when a feature or algorithm needs dependencies that should not be installed for every |pruna| user, for example because they are heavy, platform-specific, or only needed for one backend.
+Define the dependency group in ``pyproject.toml`` under ``[project.optional-dependencies]``.
+For algorithms, set ``required_install`` on the algorithm class, for example ``required_install = "uv pip install 'pruna[my-extra]'"``; this is used in the import error shown when the extra is missing.
+Finally, register a matching ``requires_<extra>`` marker in ``tests/conftest.py``, mark tests that need the extra, and add the extra to the CI matrix with its install option and marker filter, for example ``extras: "--extra my-extra"`` and ``mark_filter: "requires_my_extra"``.
+
 **If you used Option B (pip/conda):**
 
 .. code-block:: bash