message: Document AVIF support in README
diff: @@ -29,6 +29,7 @@ https://docs.rs/image
  | ICO | Yes | Yes |
  | TIFF | Baseline(no fax support) + LZW + PackBits | RGB(8), RGBA(8), Gray(8) |
  | WebP | Lossy(Luma channel only) | No |
  +| AVIF | No | Lossy |
  | PNM | PBM, PGM, PPM, standard PAM | Yes |
  | DDS | DXT1, DXT3, DXT5 | No |
  | TGA | Yes | RGB(8), RGBA(8), BGR(8), BGRA(8), Gray(8), GrayA(8) |
  