Side notes for users not seeing matching output

[ammkrn]

I'm trying to work through the examples having used the official installer, and there are some minor differences between what's in the guide and what you get with an official install. I do not think the authors/maintainers should have to do extra work to support people who aren't following directions, but I also think many would-be readers will be working off an official install (e.g. someone trying out nixos in a VM), so as long as the differences can be patched with small footnotes it probably doesn't hurt.

The first one is that you need to activate flakes/nix command, the second is that you don't get a visual cue when running the nix develop example in entry 3 (this closes #315 which had the same issue).