PR #283 : Adding images in alerts

[guillaume...@gmail.com]

Voting started on a PR to add images into GTFS-rt alerts as an experimental field. Service alerts sometime can benefit of having an image next to the alert description to enhance the comprehension of the alert.

https://github.com/google/transit/pull/283

Thanks, 

[guillaume...@gmail.com]

This PR was modified significantly so voting was paused before the previous vote was concluded. 

We’re now ready to start an other vote to add images in GTFS-rt alerts. Voting just started on GitHub : https://github.com/google/transit/pull/283