Add release notes for `subprojects packagefiles`.

docs/markdown/snippets/subprojects_packagefiles.md

@@ -0,0 +1,11 @@
+## New `subprojects packagefiles` subcommand
+
+It is now possible to re-apply `meson.build` overlays (`patch_filename` or
+`patch_directory` in the wrap ini file) after a subproject was downloaded and
+set up, via `meson subprojects packagefiles --apply <wrap-name>`.
+
+It is also possible for `patch_directory` overlays in a `[wrap-file]`, to copy
+the packagefiles out of the subproject and back into `packagefiles/<patch_directory>/`
+via `meson subprojects packagefiles --save <wrap-name>`. This is useful for
+testing an edit in the subproject and then saving it back to the overlay which
+is checked into git.