doc: Old fee_estimates.dat are discarded by 0.15.0

doc/release-notes.md

@@ -21,13 +21,17 @@ How to Upgrade
 
 If you are running an older version, shut it down. Wait until it has completely
 shut down (which might take a few minutes for older versions), then run the 
-installer (on Windows) or just copy over /Applications/Bitcoin-Qt (on Mac)
+installer (on Windows) or just copy over `/Applications/Bitcoin-Qt` (on Mac)
 or `bitcoind`/`bitcoin-qt` (on Linux).
 
 The first time you run version 0.15.0, your chainstate database will be converted to a
 new format, which will take anywhere from a few minutes to half an hour,
 depending on the speed of your machine.
 
+The file format of `fee_estimates.dat` changed in version 0.15.0. Hence, a
+downgrade from version 0.15.0 or upgrade to version 0.15.0 will cause all fee
+estimates to be discarded.
+
 Note that the block database format also changed in version 0.8.0 and there is no
 automatic upgrade code from before version 0.8 to version 0.15.0. Upgrading
 directly from 0.7.x and earlier without redownloading the blockchain is not supported.