README: Mark GIF as work in progress

Author: UlrichEckhardt

README.md

@@ -29,7 +29,7 @@ Anything that's not listed below is not currently planned to be implemented.
 - :white_check_mark: JPEG
 - :white_check_mark: PNG
 - :white_check_mark: HEIC/HEIF
-- :x: GIF
+- :construction: GIF
 - :x: TIFF
 - :x: WebP