Proposal: add SPECIAL_EVENT Cause to GTFS-realtime Service Alerts

[Cody Kraatz]

I've created an issue about this on Github after a discussion on MobilityData Slack: https://github.com/google/transit/issues/576

Thank you for taking a look and weighing in before this becomes a full-fledged Pull Request up for a vote.

Cody

CEO, SimplifyTransit