disambiguate `fh login` from `determinate-nixd login`

I was majorly confused that I apparently had to run *both* (or just the ladder) of the above commands to get my local machine to use flakehub (cache).

To a nix noob like myself, some more explanation of what fh and what determinate nix do would be helpful.

Apologies for placing this in this repo; couldn't find the overall docs repo (see #19).