Improve documentation with configuration informaton

From the existing documentation it is not clear to me if `flogger` is intended mostly as a replacement for the logging api (`slf4j`) or also for the logging backend (`logback`/`log4j`), or both.

The documentation focuses quite convincingly on why this is a better API, but does not seem to give any information about how to configure the backend (log appenders, formatting, MDC etc.). It also doesn't mention anything about whether it's recommended to use an alternate backend, and if so, how it would be accomplished.

Perhaps I've missed something, but could you possibly either point me to the existing docs i missed, or clarify some of the above? Thanks much!