Explain that ipywidgets is part of jupyter-widgets software subproject in docs

Author: paddymul

docs/source/contributing.md

@@ -1,5 +1,7 @@
 # Contributing
 
+The ipywidgets package is under the [Jupyter-Widgets](https://github.com/jupyter-widgets) [software subproject](https://jupyter.org/governance/software_subprojects.html).
+
 We appreciate contributions from the community.
 
 We follow the [IPython Contributing Guide](https://github.com/ipython/ipython/blob/main/CONTRIBUTING.md)