Update of db-sync readme

Nobody has claimed this yet.

I encountered some problems with setting up notiffications for fails on some of my projects that i set up with db-sync extension.

There were some errors **BUT I MANAGED TO DO IT.** Here are some of my key points after tests

Nethertheless, documentation wasn't 1:1 what i had to configure:
- **--log-verbosity** parameters aren't any longer errors or messages **but**: {DEBUG,INFO,WARNING,ERROR,FATAL,CRITICAL}. Yet if i choose ERROR it still displays to me every message from db-sync (every pull every few seconds even if there wasn't any errror in sync)
- if i didn't set up **smtp_username** there was this error on startup of a docker container: dynaconf.vendor.box.exceptions.BoxKeyError: "'DynaBox' object has no attribute 'smtp_username'". In documentation this parameter is optional so it is not needed to be written even in yaml file (in my opinion) and if i didn't even write this parameter there, i had this error
- **use_tls** parameter isn't even recognised
- **minimal_email_interval** by default is set i think to 4 seconds not 4 hours as it is said in documentation. I got new e-mail about sync failure every few seconds.

Start with the db-sync README and the notification configuration for --log-verbosity, smtp_username, use_tls, and minimal_email_interval. Reproduce the reported setup in the Docker container and compare the observed behavior with the documented options. Done means the README accurately describes supported parameters, optional values, logging behavior, and the verified default interval.

Written by the indexing model from the issue text.