Improve documentation for slf4j usage

[hrgdavor]

Maybe I missed it, but had rally hard time integrating slf4js. I added correct dependencies, but default logger factory was used.

After debugging and diggind through source, I found out that I need to add system property: -Dflogger.backend_factory=com.google.common.flogger.backend.slf4j.Slf4jBackendFactory#getInstance

I have not found mention of this anywhere. I would be nice to have it mentioned in the docs.

[clearnote01]

Maybe this can be expanded to include log4j, which has the similar requirement to set system property, -Dflogger.backend_factory=com.google.common.flogger.backend.log4j.Log4jBackendFactory#getInstance

This as far as I could find wasn't mentioned in either https://google.github.io/flogger or README.md.

Also is the documentation update in progress? I would love to help if it's required.