Add release notes entry

doc/release-notes.md

@@ -19,3 +19,49 @@ estimate.
 Statistics used to estimate fees and priorities are saved in the
 data directory in the 'fee_estimates.dat' file just before
 program shutdown, and are read in at startup.
+
+Double-Spend Relay and Alerts
+=============================
+VERY IMPORTANT: *It has never been safe, and remains unsafe, to rely*
+*on unconfirmed transactions.*
+
+Relay
+-----
+When an attempt is seen on the network to spend the same unspent funds
+more than once, it is no longer ignored.  Instead, it is broadcast, to
+serve as an alert.  This broadcast is subject to protections against
+denial-of-service attacks.
+
+Wallets and other bitcoin services should alert their users to
+double-spends that affect them.  Merchants and other users may have
+enough time to withhold goods or services when payment becomes
+uncertain, until confirmation.
+
+Bitcoin Core Wallet Alerts
+--------------------------
+The Bitcoin Core wallet now makes respend attempts visible in several
+ways.
+
+If you are online, and a respend affecting one of your wallet
+transactions is seen, a notification is immediately issued to the
+command registered with `-respendnotify=<cmd>`.  Additionally, if
+using the GUI:
+ - An alert box is immediately displayed.
+ - The affected wallet transaction is highlighted in red until it is
+   confirmed (and it may never be confirmed).
+
+A `respendsobserved` array is added to `gettransaction`, `listtransactions`,
+and `listsinceblock` RPC results.
+
+Warning
+-------
+*If you rely on an unconfirmed transaction, these change do VERY*
+*LITTLE to protect you from a malicious double-spend, because:*
+
+ - You may learn about the respend too late to avoid doing whatever
+   you were being paid for
+ - Using other relay rules, a double-spender can craft his crime to
+   resist broadcast
+ - Miners can choose which conflicting spend to confirm, and some
+   miners may not confirmg the first acceptable spend they see
+