[Revision history] Move Schedule Revision history out of the spec ame…

…ndment process and add the realtime revision history to this repo (#467)

* Add realtime revision history and remove schedule revision history

* Add revision history

gtfs-realtime/spec/en/revision-history.md

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+### Revision History
+
+#### March 2024
+
+* Adopted Trip-Modifications. See [discussion](https://github.com/google/transit/pull/403).
+
+### November 2022
+
+* Added support for DELETED trips. See [discussion](https://github.com/google/transit/pull/352).
+
+### July 2022
+
+* Add cause_detail and effect_detail. See [discussion](https://github.com/google/transit/pull/332)
+* Added ability to specify a wheelchair_accessible value in a TripUpdate.VehicleDescriptor. See [discussion](https://github.com/google/transit/pull/340).
+
+### September 2021
+
+* Feature/image in alerts. See [discussion](https://github.com/google/transit/pull/283).
+
+### August 2021
+
+* Add GTFS-NewShapes as experimental. See [discussion](https://github.com/google/transit/pull/272).
+
+### April 2021
+
+* Add departure_occupancy_status to TripUpdate. See [discussion](https://github.com/google/transit/pull/260).
+
+### February 2021
+
+* Clarification of GTFS Realtime occupancy descriptions. See [discussion](https://github.com/google/transit/pull/259).
+
+### September 2020 
+
+* Support multi-car crowding. See [discussion](https://github.com/google/transit/pull/237).
+
+### April 2020
+
+* Support stop assignments. See [discussion](https://github.com/google/transit/pull/219).
+
+### July 2020
+
+* Support DUPLICATED trips. See [discussion](https://github.com/google/transit/pull/221).
+* Alert tts_header_text, tts_description_text no longer experimental. See [discussion](https://github.com/google/transit/pull/229).
+* Label GTFS-RT ADDED trips as not fully specified. See [discussion](https://github.com/google/transit/pull/230).
+
+#### April 2020
+
+* Mark SeverityLevel as final. See [discussion](https://github.com/google/transit/pull/214).
+* Add occupancy_percentage. See [discussion](https://github.com/google/transit/pull/213).
+
+#### March 12, 2020
+
+* Recommend providing TripUpdate predictions for the next trip in block. See [discussion](https://github.com/google/transit/pull/206).
+
+#### August 2019
+
+* Document that trip_updates are not required to occur in feed in block-order. See [discussion](https://github.com/google/transit/pull/176).
+* Add StopTimeUpdate.ScheduleRelationship UNSCHEDULED value. See [discussion](https://github.com/google/transit/pull/173).
+
+#### May 2019
+
+* Add accessibility issue alert effect. See [discussion](https://github.com/google/transit/pull/164).
+
+#### February 2019
+
+* Add NO_EFFECT effect option for GTFS-realtime service alert. See [discussion](https://github.com/google/transit/pull/137).
+* Add new optional field SeverityLevel to Service Alerts feed. See [discussion](https://github.com/google/transit/pull/136).
+* Add new optional fields for Text-to-Speech functionality in Service Alerts feed. See [discussion](https://github.com/google/transit/pull/135).
+
+#### April 2018
+
+* Remove requirement for stop_time_update arrival AND departure for SCHEDULED trips. See [discussion](https://github.com/google/transit/pull/165).
+
+#### August 2017
+
+* Define semantic cardinality for GTFS-realtime fields. See [discussion](https://github.com/google/transit/pull/64).
+
+#### January 30, 2015
+
+* Added Protocol Buffer extension namespace to all remaining GTFS-realtime messages that didn't already have one (such as `FeedMessage` and `FeedEntity`).
+
+#### January 28, 2015
+
+* Added experimental field `delay` to `TripUpdate` ([discussion](https://groups.google.com/forum/#!topic/gtfs-realtime/NsTIRQdMNN8)).
+
+#### January 16, 2015
+
+* Update description of `TripDescriptor.start_time`.
+
+#### January 8, 2015
+
+* Defined experimental enum `OccupancyStatus`.
+* Added experimental field `occupancy_status` to `VehiclePosition` ([discussion](https://groups.google.com/forum/#!topic/gtfs-realtime/_HtNTGp5LxM)).
+
+#### May 22, 2014
+
+* Updated description of `ScheduleRelationship` enum in `StopTimeUpdate` message ([discussion](https://groups.google.com/forum/#!topic/gtfs-realtime/77c3WZrGBnI)).
+* Removed REPLACEMENT from `ScheduleRelationship` enum values in `TripDescriptor` message ([discussion](https://groups.google.com/forum/#!topic/gtfs-realtime/77c3WZrGBnI)).
+
+#### Oct 12, 2012
+
+* Added timestamp field to `TripUpdate` message.
+
+#### May 30, 2012
+
+* Added specific details about Extensions to the specification.
+
+#### November 30, 2011
+
+* Added Protocol Buffer extension namespace to key GTFS-realtime messages to facilitate writing extensions to the spec.
+
+#### October 25, 2011
+
+* Updated documentation to clarify that `alert`, `header_text` and `description_text` are both plain-text values.
+
+#### August 20, 2011
+
+* Updated documentation to clarify semantics of the `TimeRange` message.
+
+#### August 22, 2011
+
+* Initial version.