README: Mark TIFF as supported

Author: UlrichEckhardt

README.md

@@ -30,7 +30,7 @@ Anything that's not listed below is not currently planned to be implemented.
 - :white_check_mark: PNG
 - :white_check_mark: HEIC/HEIF
 - :x: GIF
-- :x: TIFF
+- :construction: TIFF (Basic support exists, but it is mostly unproven)
 - :x: WebP
 
 ### Supported assertions