Add guidance on how to make data_id unique

[6a6d74]

The WIS2 Notification Message spec says that data_id must be unique, but there's no guidance on how data publishers might do this.

WIS2box made a choice to append a locally unique "file" identifier onto (most of) the topic hierarchy, for example:

wis2/hk-hko-swic/data/core/weather/advisories-warnings/ury/00-6804254d609d389f811d069a0dcbc9b1.xml

(note that the origin\a\ is not included; we shouldn't discriminate between cache and origin for the data_id, and don't need the topic-hierarchy version identifier a).

Useful to add a note to the Guide on choosing a unique identifier for the data_id and illustrate how WIS2box does it, as above.