Adding a line to Changelog about FFV1 version 1.3 now being stable

FFV1 version 1.3 is non-experimental as of Aug 26, 2013. Adding that to the changelog.

Signed-Off By: dericed <dave@dericed.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Changelog

@@ -19,6 +19,7 @@ version <next>
 - phase filter ported from libmpcodecs
 - w3fdif filter
 - Opus support in Matroska
+- FFV1 version 1.3 is stable and no longer experimental
 
 
 version 2.0: