add release note for sendmany output shuffling

Author: instagibbs

doc/release-notes.md

@@ -73,6 +73,7 @@ RPC changes
 - Wallet `listreceivedbylabel`, `listreceivedbyaccount` and `listunspent` RPCs
   add `label` fields to returned JSON objects that previously only had
   `account` fields.
+- `sendmany` now shuffles outputs to improve privacy, so any previously expected behavior with regards to output ordering can no longer be relied upon.
 
 External wallet files
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