Nix: Note the nix: option

Author: grahamc

user/languages/nix.md

@@ -46,7 +46,14 @@ The following command line tools are available in the Nix environment:
 
 ## Default Nix Version
 
-This installs the current version of Nix using https://nixos.org/nix/install. In the future, it may be possible to configure different versions with `.travis.yml`.
+This installs Nix 2.0.4 using https://nixos.org/releases/nix-2.0.4/install. You may specify a different version of Nix installer with the `nix:` key in your `.travis.yml`:
+
+```yml
+language: nix
+nix: 2.0.4
+```
+
+> Note: at this time not all versions will successfully install, and you can only specify a specific version equal to or greater than 2.0.4.
 
 ## Default Target