Add release notes about unified bitcoin-qt and bitcoind persistent settings

If a bitcoind setting like pruning, port mapping, or a network proxy is enabled
in the GUI, it will now be stored in the bitcoin persistent setting file and
shared with bitcoind, instead of being stored as Qt settings backed by the
windows registry or platform specific config files.

Author: ryanofsky

doc/release-notes-15936.md

@@ -0,0 +1,15 @@
+GUI changes
+-----------
+
+Configuration changes made in the bitcoin GUI (such as the pruning setting,
+proxy settings, UPNP preferences) are now saved to `<datadir>/settings.json`
+file rather than to the Qt settings backend (windows registry or unix desktop
+config files), so these settings will now apply to bitcoind, instead of being
+ignored.
+
+Also, the interaction between GUI settings and `bitcoin.conf` settings is
+simplified. Settings from `bitcoin.conf` are now displayed normally in the GUI
+settings dialog, instead of in a separate warning message ("Options set in this
+dialog are overridden by the configuration file: -setting=value"). And these
+settings can now be edited because `settings.json` values take precedence over
+`bitcoin.conf` values.