add comment on nerf.py

[sabin5105]

I just added some python comment not editing any source of yours. possibly to make it easy to understand for beginners

I already know you have a full explanation on medium post but If there's some much more comment on the code, it's gonna be straightforward

please feel free to reject my suggestion or not

[MaximeVandegar]

Thank you for your suggestion, and for the great comments!