Document log level convention

[maxfischer2781]

In addition to a convention for logger naming, the discussion in MatterMiners/tardis#137 and MatterMiners/tardis#146 suggests to add a convention for log levels as well.

Log Levels: We have the standard Python log levels. IMO these should be:

• CRITICAL: COBalD or TARDIS can't go on anymore and shuts down
• ERROR: Basically unused.
• WARNING: Recoverable errors. Dropped connections, timeouts, the like. Default for monitoring health when you know things generally work.
• INFO: High-level description of what goes on. Default for monitoring behaviour when you want to understand why things happen.
• DEBUG: Low-level/Implementation details. If things go wrong and you want to report them.