There is no real changelog, but you can look at the following things:
The release changelog, which should contain most user-visible changes, including new features and bug fixes:
The git log, which is the "real" changelog
DOCSsub directoryon the git repository, which document API and user interface changes (the latter usually documents breaking changes only, rather than additions).
DOCSsub directory on the git repository, which used to be in place of this section. It documents some changes that happened since mplayer2 forked off MPlayer. (Not updated anymore.)