doc: Add release notes for -includeconf

Author: kallewoof

doc/release-notes-pr10267.md

@@ -0,0 +1,13 @@
+Changed command-line options
+----------------------------
+
+- `-includeconf=<file>` can be used to include additional configuration files.
+  Only works inside the `bitcoin.conf` file, not inside included files or from
+  command-line. Multiple files may be included. Can be disabled from command-
+  line via `-noincludeconf`. Note that multi-argument commands like
+  `-includeconf` will override preceding `-noincludeconf`, i.e.
+
+    noincludeconf=1
+    includeconf=relative.conf
+
+  as bitcoin.conf will still include `relative.conf`.