Add maintainer for WebVTT demuxer and muxer

Matthew J Heaney is added to the list of ffmpeg maintainers, for the
WebVTT demuxer and muxer.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/decoders: Document libcelt

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