message: Annotate C libraries in README.md
diff: @@ -26,14 +26,17 @@ All image processing functions provided operate on types that implement the `Gen
  | BMP | Yes | Rgb8, Rgba8, Gray8, GrayA8 |
  | ICO | Yes | Yes |
  | TIFF | Baseline(no fax support) + LZW + PackBits | Rgb8, Rgba8, Gray8 |
  -| WebP | Yes | Rgb8, Rgba8 |
  -| AVIF | Only 8-bit | Lossy |
  +| WebP | Yes | Rgb8, Rgba8 \* |
  +| AVIF | Only 8-bit \*\* | Lossy |
  | PNM | PBM, PGM, PPM, standard PAM | Yes |
  | DDS | DXT1, DXT3, DXT5 | No |
  | TGA | Yes | Rgb8, Rgba8, Bgr8, Bgra8, Gray8, GrayA8 |
  | OpenEXR | Rgb32F, Rgba32F (no dwa compression) | Rgb32F, Rgba32F (no dwa compression) |
  | farbfeld | Yes | Yes |
  +- \* Requires the `webp-encoder` feature, uses the libwebp C library.
  +- \*\* Requires the `avif-decoder` feature, uses the libdav1d C library.
  +
  ### The [`ImageDecoder`](https://docs.rs/image/*/image/trait.ImageDecoder.html) and [`ImageDecoderRect`](https://docs.rs/image/*/image/trait.ImageDecoderRect.html) Traits
  All image format decoders implement the `ImageDecoder` trait which provide
  