Add a note about (unsquashfs -o)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

sif/load.go

@@ -180,6 +180,9 @@ func convertSIFToElements(ctx context.Context, sifImage *sif.FileImage, tempDir
 	if err != nil {
 		return "", nil, fmt.Errorf("looking up rootfs from SIF file: %w", err)
 	}
+	// TODO: We'd prefer not to make a full copy of the file here; unsquashfs ≥ 4.4
+	// has an -o option that allows extracting a squashfs from the SIF file directly,
+	// but that version is not currently available in RHEL 8.
 	if err := func() error { // A scope for defer
 		f, err := os.Create(squashFSPath)
 		if err != nil {