Add a point about disabling logs in the docs

[scarlehoff]

As per issue #33, add a mention to the possibility of disabling the logs when using pdfflow.

import logging
logger_pdfflow = logging.getLogger('pdfflow')
logger_pdfflow.setLevel(logging.WARNING)

It might make sense to have it also as a environment variable because when pdfflow is used from C, Fortran, etc, changing the log level won't be this easy.