[doc] release note for min feerate changes

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+Mining and Transaction Relay Policy
+=========================
+
+The minimum block feerate (`-blockmintxfee`) has been changed to 1 satoshi per kvB. It can still be changed using the
+configuration option.
+
+The default minimum relay feerate (`-minrelaytxfee`) and incremental relay feerate (`-incrementalrelayfee`) have been
+changed to 100 satoshis per kvB. They can still be changed using their respective configuration options, but it is
+recommended to change both together if you decide to do so.
+
+Other minimum feerates (e.g. the dust feerate, the minimum returned by the fee estimator, and all feerates used by the
+wallet) remain unchanged. The mempool minimum feerate still changes in response to high volume but more gradually, as a
+result of the change to the incremental relay feerate.
+
+Note that unless these lower defaults are widely adopted across the network, transactions created with lower fee rates
+are not guaranteed to propagate or confirm. The wallet feerates remain unchanged; `-mintxfee` must be changed before
+attempting to create transactions with lower feerates using the wallet.