Provide better documentation for using custom test logger in dotnet test and make the tool produce more diagnostics.

[voroninp]

I believe things can be improved...a lot.

There is no schema for runsettings file. At least I could not find it.
There are no examples of how to reference an assembly with custom test logger placed somewhere in directory tree of the repository.
There's no guidance which framework I must/can target (netstandatd2.0 , net x.0 ?) when implementing my test logger.

The message Could not find a test logger with AssemblyQualifiedName, URI or FriendlyName 'TestLogger, Json.TestLogger'. is pretty much useless. It does not tell me what I need to do to fix the problem.
There's no information on how probing was done.