Update of db-sync readme #146

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.