avif in readme

README.md

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-A build script for preparing files for production. It handles CSS and JavaScript minification, Sass compilation, and image conversion to WebP.
+A build script for preparing files for production. It handles CSS and JavaScript minification, Sass compilation, and image conversion to WebP and AVIF.
 
-WebP conversion requires libwebp.
+WebP conversion requires libwebp. AVIF conversion requires avifenc.
 
 ## Installation