Changelog: Explain why C++ support is added, and that its optional

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

Changelog

@@ -42,7 +42,7 @@ easier to use. The changes are:
     * -intra option was removed, it's equivalent to -g 0.
 - XMV demuxer
 - Windows Media Image decoder
-- C++ Support
+- Optional C++ Support (needed for libstagefright)
 - H.264 Decoding on Android via Stagefright