Writing a notebook contribution guide

As discussed in #321, it is desired to keep the jupyter notebooks clean for version control.
Therefore, a contribution Guide for the notebooks could be written.
This guide could also be in form of a notebook and live at https://simpleitk-notebooks.readthedocs.io/.
Things that come into my mind for notebooks right now:

* They should get formatted by the `black` formatter.
* Metadata and empty cells should be cleaned by `nbstripout Python/*.ipynb --strip-empty-cells`

Start by reviewing issue #321 and the notebook contribution needs described there, then inspect the notebooks under Python/*.ipynb. The guide should be added to the SimpleITK Notebooks documentation at readthedocs.io and explain Black formatting plus the nbstripout command with empty-cell stripping; it is done when these cleanup steps are documented for contributors.

Vom Indexierungsmodell aus dem Issue-Text verfasst.