update README.md for URLSessionInstrumentation pkg

Sources/Instrumentation/URLSession/README.md

@@ -28,4 +28,5 @@ This behaviour can be modified or augmented by using the optional callbacks defi
 
 `receivedError: ((Error, DataOrFile?, HTTPStatus, Span) -> Void)?` -  Called after an error is received,  it allows to add extra information to the Span
 
+`baggageProvider: ((URLRequest, Span) -> (Baggage)?)?`: Provides baggage instance for instrumented requests that is merged with active baggage. The callback receives URLRequest and Span parameters to create dynamic baggage based on request context. The resulting baggage is injected into request headers using the configured propagator.