Document NASM input/output convention

[Voileexperiments]

Currently this information is not documented anywhere, and it's always up to each kata's whim to tell users this information.

Which registers are used for input and output? What is the argument order?

[hobovsky]

This can be somewhat deduced, as the command line used to compile (see docs) suggests 64-bit output executable, and there's very few 64-bit calling conventions . Even for me it was quite easy to narrow them down to so-called "System V AMD64 ABI" as it seems to be the only one (or one of the few) commonly used for 64bit Linux programs.

Of course, it could be helpful to add this information to NASM doc page.

(Sorry but I am on a mobile and cannot post links now for some strange reason, I will fill them in later today).