cjpeg: Adjust claims RE: image quality settings

Quality values > 95 are not useless.  They just may not provide as good
of a size vs. perceptual quality tradeoff as lower quality values.  This
also displays the default quality value in the cjpeg usage.

Closes #39

cjpeg.c

@@ -155,7 +155,8 @@ usage (void)
 #endif
 
   fprintf(stderr, "Switches (names may be abbreviated):\n");
-  fprintf(stderr, "  -quality N[,...]   Compression quality (0..100; 5-95 is useful range)\n");
+  fprintf(stderr, "  -quality N[,...]   Compression quality (0..100; 5-95 is most useful range,\n");
+  fprintf(stderr, "                     default is 75)\n");
   fprintf(stderr, "  -grayscale     Create monochrome JPEG file\n");
   fprintf(stderr, "  -rgb           Create RGB JPEG file\n");
 #ifdef ENTROPY_OPT_SUPPORTED