I wanted to list the different texlive collections using the nix-repl, as per the [manual](https://nixos.org/nixpkgs/manual/#idm140737316065984).
It didn't work, since the nixpkgs were not loaded. Doing `:l <nixpkgs>` first resolved the problem.
This change adds the nixpkgs loading step to the manual so that the next inexperienced person don't have to figure out why it didn't work.
I tested this on NixOS unstable (16.09pre90254.6b20d5b) with nix-repl 1.11.3.