update NFT standard

cadence/contracts/standards/NonFungibleToken.cdc

@@ -8,6 +8,17 @@ The interface that all Non-Fungible Token contracts should conform to.
 If a user wants to deploy a new NFT contract, their contract should implement
 The types defined here
 
+/// Contributors (please add to this list if you contribute!):
+/// - Joshua Hannan - https://github.com/joshuahannan
+/// - Bastian Müller - https://twitter.com/turbolent
+/// - Dete Shirley - https://twitter.com/dete73
+/// - Bjarte Karlsen - https://twitter.com/0xBjartek
+/// - Austin Kline - https://twitter.com/austin_flowty
+/// - Giovanni Sanchez - https://twitter.com/gio_incognito
+/// - Deniz Edincik - https://twitter.com/bluesign
+///
+/// Repo reference: https://github.com/onflow/flow-nft
+
 ## `NFT` resource interface
 
 The core resource type that represents an NFT in the smart contract.
@@ -105,7 +116,7 @@ access(all) contract interface NonFungibleToken: ViewResolver {
 
         /// Gets all the NFTs that this NFT directly owns
         /// @return A dictionary of all subNFTS keyed by type
-        access(all) view fun getAvailableSubNFTS(): {Type: UInt64} {
+        access(all) view fun getAvailableSubNFTS(): {Type: [UInt64]} {
             return {}
         }
 
@@ -238,4 +249,4 @@ access(all) contract interface NonFungibleToken: ViewResolver {
             result.getIDs().length == 0: "The created collection must be empty!"
         }
     }
-}
+}