usage.txt: Restore accidentally deleted phrase

It somehow got lost when merging the jpeg-9+ documentation changes.

usage.txt

@@ -53,7 +53,8 @@ The currently supported image file formats are: PPM (PBMPLUS color format),
 PGM (PBMPLUS grayscale format), BMP, Targa, and RLE (Utah Raster Toolkit
 format).  (RLE is supported only if the URT library is available, which it
 isn't on most non-Unix systems.)  cjpeg recognizes the input image format
-automatically, with the exception of some Targa files.
+automatically, with the exception of some Targa files.  You have to tell djpeg
+which format to generate.
 
 JPEG files are in the defacto standard JFIF file format.  There are other,
 less widely used JPEG-based file formats, but we don't support them.