Bug 1823953 - Update ffvpx's README_MOZILLA to mention that it can use libvorbis now. r=alwu

Differential Revision: https://phabricator.services.mozilla.com/D175154

media/ffvpx/README_MOZILLA

@@ -9,10 +9,13 @@ git checkout 37cde570bc2dcd64a15c5d9a37b9fa0d78d84f9f
 
 All source files match their path from the library's source archive.
 
-Currently, we only use the vp8, vp9, av1 (libdav1d), mp3, and flac portion of
-the library. If this changes, configuration files will most likely need to be
-updated. Decoding AV1 via libdav1d is supported, although the decoder
-(libdav1d) is vendored separately, `ffvpx` only contains the code to use `libdav1d` through the `ffmpeg` API.
+Currently, we only use the vp8, vp9, av1 (via libdav1d), mp3, flac, vorbis (via
+libvorbis) portion of the library. If this changes, configuration files will
+most likely need to be updated.
+
+Decoding AV1 via libdav1d and libvorbis is supported, although the decoder
+libraries are vendored separately, `ffvpx` only contains the code to use
+those libraries through the `ffmpeg` API.
 
 The ffmpeg project recommends to use ffmpeg's tip, not a particular release.