update(os-final proj2-notes): learn how to add syscall in 4.x
update(os-final proj2-notes): learn how to add syscall in 4.x

issue sabertazimi/hust-os-2017#3
issue sabertazimi/awesome-notes#27

sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab,sabertazimi/hust-lab