Add description of nonce to Update contract-classes.adoc

This commit addresses issue #1098

components/Starknet/modules/architecture_and_concepts/pages/Smart_Contracts/contract-classes.adoc

@@ -20,6 +20,8 @@ A contract class does not necessarily have a deployed instance in Starknet.
 A contract class does not necessarily require a deployed instance in Starknet.
 ====
 
+#A contract instance has a nonce, the value of which is the number of transactions originating from this address plus 1. For example, when you deploy an account with a `DEPLOY_ACCOUNT` transaction, the nonce of the account contract in the transaction is `0`. After the transaction, until the account contract sends its first transaction, the nonce is `1`.#
+
 == Working with classes
 
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