Add safe duration fields to trips.txt to provide better flexible trip time estimates (#598)

Add safe_duration fields to trips.txt
Update reference.md
Formatting update for safe duration fields

Each field gets its own row and description, extended usage guidance in its own separate section below table.

General Transit Feed Specification (GTFS)

The General Transit Feed Specification (GTFS) is an Open Standard used to distribute relevant information about transit systems to riders. It allows public transit agencies to publish their transit data in a format that can be consumed by a wide variety of software applications. Today, the GTFS data format is used by thousands of public transport providers. GTFS consists of two formats: GTFS Schedule and GTFS Realtime.

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GTFS relies on community contributions. Please help us capture GTFS developments by sharing your updates for our next digest to: specifications@mobiltydata.org

Useful Resources

Getting Started with GTFS

Visit GTFS.org to learn more about GTFS
- An overview of GTFS Schedule
- An overview of GTFS Realtime

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Join Slack and introduce yourself to the community.
Join GTFS Changes and GTFS Realtime Google Groups to stay informed on development.

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