feh(1): Add --cache-thumbnails option

feh.1

@@ -260,6 +260,9 @@ below each thumbnail.
 .B -t, --thumbnails
 Same as Index mode, but the thumbnails are clickable image launchers.
 .TP 5
+.B --cache-thumbnails
+Enable thumbnail caching (in ~/.thumbnails)
+.TP 5
 .B --bg-tile FILE
 .TP 5
 .B --bg-center FILE