Extend readme

src/ImageSharp/Formats/Heif/Av1/Readme.md

@@ -49,4 +49,19 @@ The smallest unit in the frame. It determines the parameters for an area of 4 by
 
 # References
 
+[AV1 embedded in HEIF](https://aomediacodec.github.io/av1-isobmff)
+
 [AV1 specification](https://aomediacodec.github.io/av1-spec/av1-spec.pdf)
+
+[AVIF specification](https://aomediacodec.github.io/av1-avif)
+
+[AV1/AVIF reference implementation](http://gitlab.com/AOMediaCodec/SVT-AV1)
+
+[AOM's original development implementation](https://github.com/AOMediaCodec/libavif)
+
+# Test images
+
+[Netflix image repository](http://download.opencontent.netflix.com/?prefix=AV1/)
+
+[AVIF sample images](https://github.com/link-u/avif-sample-images)
+

src/ImageSharp/Formats/Heif/Readme.md

@@ -4,6 +4,4 @@
 
 [HEIF reference implementation from Nokia](https://github.com/nokiatech/heif)
 
-[AVIF reference implementation](http://gitlab.com/AOMediaCodec/SVT-AV1)
-
 [Apple's metadata syntax in HEIC images](http://cheeky4n6monkey.blogspot.com/2017/10/monkey-takes-heic.html)