m64 format documentation

[lezg_g]

Through reverse-engineering of the SM64 audio library, I figured some more details of the m64 format.

The hack64 forum software doesn't seem to support tables, but here's an external markdown doc: https://hackmd.io/opEB-OmxRa26P8h8pA-x7w as well as a decoder: https://gist.github.com/simonlindholm/30...22c291f878

The list of commands should be complete, though some of the descriptions are a bit vague. I could look closer into the code if there's some particular aspect that people are interested in.

Man, thank you! this helps a lot! I added some commands in seq64 and works really great.