WIP: added text allowing "Z" for timezone offset.

[ChrisBarker-NOAA]

See issue #584 for discussion of these changes.

NOTE: I think there's consensus about allowing "Z" as a timezone offset (already allowed by udunits)

There is not yet consensus on whether to include a recommendation that a zero or Z be included for UTC times.

Release checklist

** Will do this once we've all agreed **

• Authors updated in cf-conventions.adoc? Add in two places: on line 3 and under .Additional Authors in About the authors.
• Next version in cf-conventions.adoc up to date? Versioning inspired by SemVer.
• history.adoc up to date?
• Conformance document up to date?

For maintainers

After the merge remember to delete the source branch.
Tags are set at the conclusion of the annual meeting; until then, main always is a draft for the next version.