Added clarification on how to perform an audit of history.

Signed-off-by: Richard Esplin <[email protected]>

text/0162-frozen-ledgers/README.md

@@ -116,6 +116,8 @@ If a ledger is added (so its root hash is expected by the audit ledger), but it
 
 But the third property will not be preserved if there is a history of transactions on a ledger and then the ledger is removed. This is because the LEDGERS_FREEZE transaction only stores the most recent root hash, and not the entire history of root hashes, so that history can not be recreated after the plugin ledger is removed. Instead, the frozen ledger should be retained in its read-only state.
 
+Note that Indy Node does not include functionality to perform an audit of the ledger history based on the original validation rules. Indy transaction handlers perform validation on new transactions, but not during the catchup process. This is because the transaction is already on the ledger (it has a state signature), so we know that validation was already performed and we don't need to do it again. An audit of the history would have to be done outside of Indy Node by replaying the transactions using the original validation logic, using the audit ledger to compare the state among the various ledgers. Freezing a ledger does not change this process, but dropping a frozen ledger that contains historical transactions would eliminate data that would be necessary to perform a complete audit of the ledger history.
+
 
 ## Rationale and alternatives
 [alternatives]: #alternatives