Better description of different levels

[erickhun]

following @colinscape comment. This PR is inspired from the Monolog comments and give better description of the different logs level to use.

I've tried to add different examples based on some use case i've seen when integrating BuffLog to some of our services.

Levels	Use case	Examples
DEBUG	Information used for interactive investigation, with no long-term value.	Printing function names, steps inside a function.
INFO	Interesting events. Track the general flow of the application.	User logs in, SQL logs, worker process/delete a message...
NOTICE	Uncommon events. This is the default verbosity level.	Missing environment variables, page redirection, pod starting/restarting/terminating, retrying to query an API...
WARNING	Exceptional occurrences that are not errors. Undesirable things that are not necessarily wrong.	Use of deprecated APIs, poor use of an API, unauthorized access, pod restart because of memory limit ...
ERROR	Runtime errors. Highlight when the current flow of execution is stopped due to a failure.	Exceptions messages, incorect credentials or permissions...
CRITICAL	Critical conditions. Describe an unrecoverable application, system crash, or a catastrophic failure that requires immediate attention.	Application component unavailable, unexpected exception. entire website down, database unavailable ...

@colinscape @esclapes Would that be enough for engineers to know which level to use?

[esclapes]

This seems both clear and useful to me @erickhun 🌮 👍

[colinscape]

Looks great to me - definitely gives a lot more clarity on what is what! 🌟