As I understand it, one of the libretro team needs to create the bounty in order for it to appear in the main bounty list. @Twinaphex if it’s acceptable to you, would you be willing to create a Bountysource listing along these lines?
The bounty is to create a new MAME 2014 core doc file in the same format as generally used in the official docs here: https://buildbot.libretro.com/docs/
It might be useful to look at the MAME 2003 doc for reference, as it is nicely comprehensive: https://buildbot.libretro.com/docs/library/mame2003/