Add a note on preserving transformation input

pkg/transform/transform.go

@@ -38,6 +38,7 @@ func NewTransformation(tranformFunctions ...TransformationFunction) Transformati
 		}
 
 		for _, message := range messages {
+			// We dereference here in order to avoid copying the input - when a transformation fails, we want the failure we create to contain the original input data
 			msg := *message // dereference to avoid amending input
 			success := &msg // success must be both input and output to a TransformationFunction, so we make this pointer.
 			var failure *models.Message