Mention in custom_plot_params.py where the 'defaults' are found

[jypeter]

The custom_plot_params.py file makes it possible to override some default plotting parameters, but it would be useful if a comment mentioned where those defaults are defined (i.e. ${CLIMAF}/climaf/plot/*_plot_params*.py files)

This is correctly explained in Add plot parameters, but a reminder in the file can't hurt. I was looking at the file before I read again the wiki page...