Autoload image-converter-add-handler

* lisp/image/image-converter.el (image-converter-add-handler):
Autoload.

etc/NEWS

@@ -1709,6 +1709,12 @@ this message for SVG and XPM.
 *** New commands: 'image-flip-horizontally' and 'image-flip-vertically'.
 These commands horizontally and vertically flip the image under point.
 
+** Images
+
++++
+*** Users can now add special image conversion functions.
+This is done via 'image-converter-add-handler'.
+
 ** Image-Dired
 
 +++

lisp/image/image-converter.el

@@ -306,6 +306,7 @@ Only suffixes that map to `image-mode' are returned."
                                           "-")))))
       "ffmpeg error when converting")))
 
+;;;###autoload
 (defun image-converter-add-handler (suffix converter)
   "Make Emacs use CONVERTER to parse image files that end with SUFFIX.
 CONVERTER is a function with one parameter, the file name.  The