Annotate C libraries in README.md

README.md

@@ -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