changelog entry: ffmpeg replaygain backend
changelog entry: ffmpeg replaygain backend

Add changelog entry for the new ffmpeg replaygain backend.

jackwilsdon/beets,sampsyo/beets,jackwilsdon/beets,shamangeorge/beets,ibmibmibm/beets,shamangeorge/beets,ibmibmibm/beets,beetbox/beets,SusannaMaria/beets,SusannaMaria/beets,beetbox/beets,jackwilsdon/beets,ibmibmibm/beets,sampsyo/beets,sampsyo/beets,jackwilsdon/beets,beetbox/beets,shamangeorge/beets,ibmibmibm/beets,shamangeorge/beets,SusannaMaria/beets,sampsyo/beets,beetbox/beets,SusannaMaria/beets