Adding a line to Changelog about Opus Matroska Support

Next release of ffmpeg will support embedding Opus in Matroska
container. Adding that to the Changelog.

Signed-Off By: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Changelog

@@ -18,6 +18,7 @@ version <next>
 - DVB teletext decoder
 - phase filter ported from libmpcodecs
 - w3fdif filter
+- Opus support in Matroska
 
 
 version 2.0: