Add best practice: In seat transfer specifics

[giocorti]

This is one of 5 PRs that incorporates a recommendation from the GTFS best practices.

This PR adds details about properly implementing in-seat transfers.

The 4 related PRs can are:

• Headsign and route name examples
• Specifics about stop_headsign
• stop_id is not rider facing
• Include arrival and departure times when possible

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[barbeau]

This is still an issue, but is probably best discussed at this most recent proposal on the main GTFS repo - google#303.