usage.txt: Restore accidentally deleted phrase

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

usage.txt

diff --git a/usage.txt b/usage.txt
index 69d4717..5abda4e 100644
--- a/usage.txt
+++ b/usage.txt
@@ -53,7 +53,8 @@
 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.