doc: Add more information in the README

And convert it to Markdown.
lavr: Update the planar check in ff_audio_convert

Leftover from fbc0b8659967ea54a8472b5f795270d38bb085dd.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
mxf: Add UID print helpers

And use it to print non-parsed ULs.
mxf: Add the UL for the MPEG2VideoDescriptor
mxf: Support AAC

Update mxf_set_audio_pts to use the container-provided information.

The UL is marked as "to be changed in the future", but the current
samples in the wild do use it.