While I too would like asciidoct support, there are other things on my list at a higher priority and I suspect this is not even on the developers’ list!
But you can convert asciidoc to git format markdown etc. going via a docbook format intermediate step. Something like
asciidoctor -b docbook -a leveloffset=+1 -o - foo.adoc | \
pandoc --atx-headers --wrap=preserve -t gfm -f docbook - > foo.md
The conversion is not perfect but good enough.