Document new support for JPEG XL

An alternative to using PPM, relies on libjxl from the JPEG.
JPEG XL is a superior modern format that supports both lossy and
lossless compression and does an excellent job at both.

The format also supports multiple frames (ie: animation), so can
be used for Cool Stuff in the future.

As an example, a large image that is 130MB in PPM format is only
6.8MB in JPEG XL format using the default quality settings of
ImageMagick (whatever they are).

src/syncterm/CHANGES

@@ -2,6 +2,7 @@ Version 1.4a
 ------------
 Re-order BBS list entry fields
 Fix various Prestel mode bugs
+Add support for JPEG XL graphics in addition to PPM
 
 Version 1.3
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