Use the new ihp-new package from nixpkgs for install instructions in …

…the docs

Guide/installation.markdown

@@ -116,6 +116,15 @@ Note that nix can gradually grow to use several GB of disk space, especially aft
 
 You can now install IHP by running:
 
+```bash
+nix-env --install ihp-new
+```
+
+
+#### Package not Found?
+
+The package has only been added to nixpkgs recently. In case you're using an older nixpkgs version, the package might not be available yet. In that case you can install `ihp-new` like this:
+
 ```bash
 nix-env -f https://downloads.digitallyinduced.com/ihp-new.tar.gz -i ihp-new
 ```