Add Nix/NixOS instructions

Author: fabaff

README.md

@@ -13,6 +13,13 @@ The package is available in the [Python Package Index](https://pypi.python.org/)
 $ pip3 install elmax --user
 ```
 
+For Nix or NixOS users is a package available. Keep in mind that the lastest
+releases might only be present in the `unstable` channel.
+
+```bash
+$ nix-env -iA nixos.python38Packages.elmax
+```
+
 ## Usage
 
 For details about the usage please check the `examples.py` file.