Documenting mask files

[DamienIrving]

#5 added code (see shapefile_masks.ipynb) that produces netCDF mask files. We should document the existence of these mask files and possibly convert shapefile_masks.ipynb to shapefile_masks.py and put it in the processing_scripts/ directory with the other processing scripts.

[DamienIrving]

cc @xenct (I'm happy to do most of the work to address this but I'll reach out if I have questions)

[xenct]

Good point, @DamienIrving. Thank you