Update CONTRIBUTING.md

Author: stephane-caron

CONTRIBUTING.md

@@ -20,5 +20,6 @@ The goal of this project is to facilitate loading and sharing of robot descripti
     - Use the ``mj_description`` suffix for an MJCF description.
 4. **Listing:** Add the description metadata to the ``DESCRIPTIONS`` dictionary in ``_descriptions.py``.
 5. **README:** Document the description's submodule name in the Descriptions section of the [README](README.md).
+    - Use an [SPDX License Identifier](https://spdx.org/licenses/) in the License column.
 6. **CHANGELOG:** Write down the new model at the top of the [changelog](CHANGELOG.md).
 7. **Testing:** Check that all unit tests are successful by ``tox``.