add recipe for detecting data subscriptions from GDC

[tomkralidis]

Thanks to @SimonElliottEUM for the discussion.

[golfvert]

It may make sense to explain that what is called here a channel is also called a topic and the information to use to subscribe to the MQTT address of the Global Broker.

[tomkralidis]

It may make sense to explain that what is called here a channel is also called a topic and the information to use to subscribe to the MQTT address of the Global Broker.

PR updated with the below note:

The channel property represents WIS2 topic which can be used to subscribe to the href property (i.e. the MQTT address) of the Global Broker (GB).