Changelog: add "version <next>:" so people dont add new things to the last release

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

Changelog

 Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
+version <next>:
+
+
 version 2.3:
 - AC3 fixed-point decoding
 - shuffleplanes filter