docs: add rider_categories to features (#1981)

Co-authored-by: cka-y <[email protected]>

Author: skalexch

docs/FEATURES.md

@@ -19,6 +19,7 @@ The validator will produce the [list of features](https://gtfs.org/getting_start
 | Base add-ons | Frequencies | One line of data in [frequencies.txt](https://gtfs.org/schedule/reference/#frequenciestxt)    | https://gtfs.org/getting_started/features/base_add-ons/#frequencies  |
 | Fares                    | Fare Products             | One line of data in [fare_products.txt](https://gtfs.org/schedule/reference/#fare_productstxt)  | https://gtfs.org/getting_started/features/fares/#fare-products |                                                                                                                      
 | Fares                    | Fare Media                | One line of data in [fare_media.txt](https://gtfs.org/schedule/reference/#fare_mediatxt) |  https://gtfs.org/getting_started/features/fares/#fare-media |
+| Fares                    | Rider Categories                | One line of data in [rider_categories.txt](https://gtfs.org/schedule/reference/#ridercategoriestxt) |  https://gtfs.org/getting_started/features/fares/#rider-categories |
 | Fares                    | Route-Based Fares         | One **network_id** value in [routes.txt](https://gtfs.org/schedule/reference/#routestxt)<br/>OR<br/>one line of data in [networks..txt](https://gtfs.org/schedule/reference/#networkstxt)   |    https://gtfs.org/getting_started/features/fares/#route-based-fares   |
 | Fares                    | Time-Based Fares          | One line of data in [timeframes.txt](https://gtfs.org/schedule/reference/#timeframestxt)   | https://gtfs.org/getting_started/features/fares/#time-based-fares  |
 | Fares                    | Zone-Based Fares          | One line of data in [areas.txt](https://gtfs.org/schedule/reference/#areastxt)   |     https://gtfs.org/getting_started/features/fares/#zone-based-fares |