Add change file

.changes/pr256.md

@@ -0,0 +1,13 @@
+---
+"cargo-packager": "patch"
+"@crabnebula/packager": "patch"
+---
+
+Automatically add an Exec arg (field code) in the `.desktop` file.
+
+This adds an `{exec_arg}` field to the default `main.desktop` template.
+This field is populated with a sane default value, based on the
+`deep_link_protocols` or `file_associations` in the `Config` struct.
+
+This allows an installed Debian package to be invoked by other
+applications with URLs or files as arguments, as expected.