compel --help: describe required parameters

TODO: describe optional ones.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

compel/src/main.c

@@ -134,7 +134,9 @@ static int usage(int rc) {
 
 	printf(
 " }\n"
-"  compel -f filename hgen\n"
+"  compel -f FILE -o FILE [...] hgen\n"
+"    -f, --file FILE		input (parasite object) file name\n"
+"    -o, --output FILE		output (header) file name\n"
 );
 
 	return rc;