User guide should suggest way to install `nix-tools`

[nomeata]

Currently https://input-output-hk.github.io/haskell.nix/user-guide/ is phrased with nix-tools already installed, and links to https://github.com/input-output-hk/nix-tools. There, installation is recommended via

nix-build -A nix-tools-all-execs

but the README at https://github.com/input-output-hk/haskell.nix recommends

nix build -f https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz nix-tools -o nt

It is unclear if there is a difference, and when to best use which, so ideally https://input-output-hk.github.io/haskell.nix/user-guide/#user-guide would just give canonical instructions.

[angerman]

Thanks for highlighting this inconsistency!

[rvl]

I added a fix in #87.