Add note about producer needed the two versions of trip updates

gtfs-realtime/spec/en/trip-modifications.md

@@ -22,6 +22,7 @@ The scheduled stop times of each replacement trip are created from those of the
 * A TripUpdate SHOULD be provided using a `ModifiedTripSelector` inside the TripUpdate's `TripDescriptor`. 
     * When the TripUpdate refers to the replacement trip, the consumer should behave as if the static GTFS would have been modified with the TripModifications (e.g. `arrival_time`, `departure_time`, `stop_sequence`, `stop_id` on replacement stops)
     * When providing a `ModifiedTripSelector`, the other fields of the `ScheduleRelationship` SHOULD be left empty, to avoid confusion by consumer that aren't looking for the `ModifiedTripSelector` value. 
+    * TripUpdate feeds providing updates with `ModifiedTripSelector` SHOULD also include a TripUpdate targetting clients that don't about TripModifications. In other words, there should be two TripUpdates : one for clients with modified trips (with `TripModifications`) and one for clients with the originial unmodified GTFS (without `TripModifications`)
     * Providing a TripUpdate with the a `ModifiedTripSelector` is the only way to create predictions at replacement stops
 * If no such TripUpdate is found, TripUpdates for the original `trip_id` will apply to the modified trip. 
     * In this case, the static GTFS information used should be from the static GTFS before any TripModifications applied.