Update Getting Started for NixOS users (#998)

* Update getting started for NixOS users

Based on the file that writes nix.conf, and visual inspection, these settings update the nix.conf on NixOS as specified.

* Better tutorial structure

responding to feedback

Author: billewanick

docs/tutorials/getting-started.md

@@ -21,6 +21,18 @@ trusted-public-keys = [...] hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNG
 substituters = [...] https://hydra.iohk.io [...]
 ```
 
+If you're running NixOS, you need to add/update the following in your `/etc/nixos/configuration.nix` files instead.
+
+```
+# Binary Cache for Haskell.nix
+nix.binaryCachePublicKeys = [
+  "hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ="
+];
+nix.binaryCaches = [
+  "https://hydra.iohk.io"
+];
+```
+
 This can be tricky to get setup properly. If you're still having trouble getting cache hits, consult the corresponding [troubleshooting section](../reference/troubleshooting#why-am-i-building-ghc).
 
 ## Scaffolding