Update timezone guidance to recommend using timezone designator in some scenarios

[nickevansuk]

Where systems have clubs that sit across timezone boundaries, the clubs timezone can be helpful to allow data users to render event times in local time.

Options:

• Add timezone field to modelling specification (e.g. something similar to http://schema.org/broadcastTimezone)
• Recommend timezone designator within existing datetime fields in RPDE specification

References:

• https://www.w3.org/International/core/2005/09/timezone.html
• https://github.com/schemaorg/schemaorg/issues/481
• New schema to support TV listings schemaorg/schemaorg#329
• https://www.youtube.com/watch?v=-5wpm-gesOY

Proposal:

• A timezone designator must only be included where providers have explicitly set the timezone, or where the system has calculated it based on provider location. Where no timezone has been selected or is available, use UTC as per previous spec.