That said, the path forward to mod your NES Classic is pretty clear but... why should you mod it at all?
Honestly if you're not very technical then i'd stay out of it entirely. You have the chance to brick your system and with how hard they've been to get bricking you NES would suck. That said, it's not hard and a couple features of Hakchi2 make it really compelling.
Forget the fact that you can load on more NES roms with Hackchi2, yeah you can do that but I haven't yet. The biggest reason to mod your SNES Classic is to allow you the ability to press down and select at the same time to return to the home menu. This is a lifesaver if the reset button is an annoying distance from you (and it is for me). Another awesome feature of using Hakchi2 is that you can copy the save states from your NES Classic to a local file for backup purposes. It's possible to accidentally (using save states w RPG's) to wipe out a ton of progress so backing up all your save files from time to time isn't a bad idea.
Hakchi2 also mods the included NES Classic kernel to support more controllers out of the box which is nice but they don't mention which ones. I imagine it's a sort of generic USB controller support which is cool if you have USB to that strange Nintendo controller format.
So even if you don't add more ROMS to your NES Classic, and i'm not sure I will, the controller home mod and save state backup is worth the price of admission alone. That said, use it at your own risk because bricking your NES wouldn't be great.
* After looking more at the Hakchi2 code it seems the author is working on SNES Classic support so i'd wait a bit until a new version is launched with SNES Classic support before you use it.