Add information where ClassMeetadata should be stored.

neps/nep-504.md

@@ -33,7 +33,7 @@ Similarly, `ContractMetadata` only defines issuer basic info.
 
 ### Class Metadata
 
-We start by defining a `ClassMetadata` which will contain all common information about tokens of the same class.
+We start by defining a `ClassMetadata` which will contain all common information about tokens of the same class. The `ClassMeetadata` object must be stored in the SBT issuer contract and accessible via `class_metadata(class: ClassId)` query.
 This will allow to avoid duplicating such information in all `TokenMetadata` and potentially allow to leave `TokenMetadata.reference` empty, if the tokens only differentiate by the class, issued_at and expires_at attributes.
 
 ```rust