Add that headsigns are recommended (#485)

gtfs/spec/en/reference.md

@@ -250,7 +250,7 @@ Primary key (`trip_id`)
 |  `route_id` | Foreign ID referencing `routes.route_id` | **Required** | Identifies a route. |
 |  `service_id` | Foreign ID referencing `calendar.service_id` or `calendar_dates.service_id` | **Required** | Identifies a set of dates when service is available for one or more routes. |
 |  `trip_id` | Unique ID | **Required** | Identifies a trip. |
-|  `trip_headsign` | Text | Optional | Text that appears on signage identifying the trip's destination to riders. Should be used to distinguish between different patterns of service on the same route.<br><br> If the headsign changes during a trip, values for `trip_headsign` may be overridden by defining values in `stop_times.stop_headsign` for specific `stop_time`s along the trip. |
+|  `trip_headsign` | Text | Optional | Text that appears on signage identifying the trip's destination to riders. This field is recommended for all services with headsign text displayed on the vehicle which may be used to distinguish amongst trips in a route.<br><br> If the headsign changes during a trip, values for `trip_headsign` may be overridden by defining values in `stop_times.stop_headsign` for specific `stop_time`s along the trip. |
 |  `trip_short_name` | Text | Optional | Public facing text used to identify the trip to riders, for instance, to identify train numbers for commuter rail trips. If riders do not commonly rely on trip names, `trip_short_name` should be empty. A `trip_short_name` value, if provided, should uniquely identify a trip within a service day; it should not be used for destination names or limited/express designations. |
 |  `direction_id` | Enum | Optional | Indicates the direction of travel for a trip. This field should not be used in routing; it provides a way to separate trips by direction when publishing time tables. Valid options are: <br><br>`0` - Travel in one direction (e.g. outbound travel).<br>`1` - Travel in the opposite direction (e.g. inbound travel).<hr>*Example: The `trip_headsign` and `direction_id` fields may be used together to assign a name to travel in each direction for a set of trips. A [trips.txt](#tripstxt) file could contain these records for use in time tables:* <br> `trip_id,...,trip_headsign,direction_id` <br> `1234,...,Airport,0` <br> `1505,...,Downtown,1` |
 |  `block_id` | ID | Optional | Identifies the block to which the trip belongs. A block consists of a single trip or many sequential trips made using the same vehicle, defined by shared service days and `block_id`. A `block_id` may have trips with different service days, making distinct blocks. See the [example below](#example-blocks-and-service-day). To provide in-seat transfers information, [transfers](#transferstxt) of `transfer_type` `4` should be provided instead. |