[Doc] Add cold-staking changes to release notes

doc/release-notes.md

@@ -44,6 +44,19 @@ Notable Changes
 
 (Developers: add your notes here as part of your pull requests whenever possible)
 
+Cold-Staking Re-Activation
+--------------------------
+PIVX Core v6.0.0 includes a fix for the vulnerability identified within the cold-staking protocol (see PR [#2258](https://github.com/PIVX-Project/PIVX/pull/2258)).
+Therefore the feature will be re-enabled on the network, via `SPORK_19`, shortly after the upgrade enforcement.
+
+#### Protocol changes
+
+A new opcode (`0xd2`) is introduced (see PR [#2275](https://github.com/PIVX-Project/PIVX/pull/2275)). It enforces the same rules as the legacy cold-staking opcode, but without allowing a "free" script for the last output of the transaction.
+This is in accord with the consensus change introduced with the "Deterministic Masternodes" update, as masternode/budget payments are now outputs of the *coinbase* transaction (rather than the *coinstake*), therefore a "free" output for the coinstake is no longer needed.
+The new opcode takes the name of `OP_CHECKCOLDSTAKEVERIFY`, and the legacy opcode (`0xd1`) is renamed to `OP_CHECKCOLDSTAKEVERIFY_LOF` (last-output-free).
+Scripts with the old opcode are still accepted on the network (the restriction on the last-output is enforced after the script validation in this case), but the client creates new delegations with the new opcode, by default, after the upgrade enforcement.
+
+
 GUI changes
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