add coding convenctions section

Document all your other conventions here. I've come across a few
deviations. Keep this section apprised so I know which deviations to
refactor to the other style.
partial/missing async doc.

added serial_async.h and async_io.h entries. unsure how serial_async
works, and how async_io relates to other APIs. something to do with ISRs
documentation

also sketch of driver controls and steering wheel alg

I assume can.h also abstracts setting up CAN pins. Also flash.h appears
unused. Also no idea what MCP49X2 is used for, possibly CAN