clarify GDC GB subscription topic (#153)

guide/sections/part2/global-services.adoc

@@ -172,6 +172,7 @@ In WIS2 Global Caches provide access to WMO core data for data consumers. This a
 * Multiple Global Discovery Catalogue instances may be deployed for resilience.
 * Global Discovery Catalogue instances operate independently of each other; each Global Discovery Catalogue instance will hold all discovery metadata records.  Global Discovery Catalogues do not need to synchronize between themselves.
 * A Global Discovery Catalogue is populated with discovery metadata records from a Global Cache instance, receiving messages about the availability of discovery metadata records via a Global Broker.
+** The subscription topic should be `+cache/a/wis2/+/metadata/#+`.
 * A Global Discovery Catalogue should connect and subscribe to more than one Global Broker instance to ensure that no messages are lost in the event of a Global Broker failure. A Global Discovery Catalogue instance will discard duplicate messages as needed.
 * A Global Discovery Catalogue will validate that a discovery metadata record identifier's `centre-id` token (see _Manual on WIS_, Volume II -Appendix F: WMO Core Metadata Profile) matches against the `centre-id` level of the topic from which it was published (see _Manual on WIS_, Volume II -Appendix D: WIS2 Topic Hierarchy), to ensure that discovery metadata is published by the authoritative organization.
 * A Global Discovery Catalogue will validate discovery metadata records against the WMO Core Metadata Profile (WCMP2).  Valid WCMP2 records will be ingested into the catalogue.  Invalid or malformed records will be discarded and reported to the Global Monitor against the centre identifier associated with the discovery metadata record.