Documentation misses key-aspects of how to use lager-logger

Current behavior

Dear development team.

To fix an issue in our software, we wanted to define minimum log-level. Unfortunately the usage-documentation misses instructions on how to do that.

That led to an unfruitful implementation-story where I discovered just at the moment when I wanted to file in a bug-report together with a fixing PR the structure of this repository and how to make use of its transformers (i.e. reconfigurableSink, redactingSink, truncatingSink). As this is a key-aspect of the design of lager-logger I personally think it should be part of the documentation. This would be highly valuable to its users and may safe them time.

Desired behavior

Key-concepts of the design of lager are explained in its usage-documentation.

Affected Version

all versions

Start with docs/usage.md and review the usage of the reconfigurableSink, redactingSink, and truncatingSink transformers mentioned in the issue. Document how to define a minimum log level and explain these key design concepts. Done means the usage documentation covers the requested lager features clearly for users.