Add release note

Author: edolstra

doc/manual/source/release-notes-determinate/changes.md

@@ -5,3 +5,5 @@ This section lists the differences between upstream Nix 2.24 and Determinate Nix
 * In Determinate Nix, flakes are stable. You no longer need to enable the `flakes` experimental feature.
 
 * In Determinate Nix, the new Nix CLI (i.e. the `nix` command) is stable. You no longer need to enable the `nix-command` experimental feature.
+
+* Determinate Nix has a setting [`json-log-path`](@docroot@/command-ref/conf-file.md#conf-json-log-path) to send a copy of all Nix log messages (in JSON format) to a file or Unix domain socket.

doc/manual/source/release-notes-determinate/rl-3.0.0.md

@@ -3,3 +3,5 @@
 * Initial release of Determinate Nix.
 
 * Based on [upstream Nix 2.26.2](../release-notes/rl-2.26.md).
+
+* New setting `json-log-path` that sends a copy of all Nix log messages (in JSON format) to a file or Unix domain socket.