Add hint to start editor in project directory

[Vlix]

I've been debugging for a few hours and getting really annoyed that HLS just didn't work out of the box, and even with tons of fiddling, it would not get the right GHC version.

Turns out, I had to start code from within the IHP project directory.

For someone who's never used Nix, this is not obvious, so it might be nice to give a warning/footnote about this in the guide when the beginner starts using their editor?