Please document that `writeToProviders` will ignore filtering rules of the .NET logging

[AntonPetrov83]

Hi!

I was resolving an issue that while AddAplicationInsightsLoggerProvider() sets the default filter rule to LogLevel.Warning (here) in the end it was ignored.

I used Serilog with UseSerilog() and writeToProviders: true.

As I understand this is because Serilog installs its own SerilogLoggerFactory which ignores .NET logging filters.

It is okay but please make it obvious from the readme documentation.

P.S. The same is true for Serilog.AspNetCore