Add documentation for setting `self.verbose` flag

[JosephShering]

[natevw]

AFAICT this must be something that comes from django-pipeline itself (maybe in https://github.com/jazzband/django-pipeline/blob/5ec680174d1897498ec9c6652a4130bc0d39dddd/pipeline/compilers/init.py#L33 ?) and so you might try their documentation.