Have you read the Django Signals Documentation and wondered, “Okay, how the heck do you actually use it? How about an example?” If so, then you’ve come to the right place.
Will demo by example adding signals for pre_save, pre_delete to raise an exception if a READ_ONLY_FILE exists to prevent the DB from changing. Then, add signals for post_save and post_delete to print the change to stdout. Using print as an example, a more useful thing would be to write it to syslog or something – say you want to log every DB change that occurs.