Writing a notebook contribution guide

[kolibril13]

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

[zivy]

There already is a contribution guide. It should be updated with these two points which have been recently added.