Add install instructions for NixOS

Author: fabaff

README.rst

@@ -22,6 +22,13 @@ On a Fedora-based system or a CentOS/RHEL machine with EPEL.
 
     $ sudo dnf -y install python3-opensensemap-api
 
+For Nix or NixOS users is a package available. Keep in mind that the lastest releases might only
+be present in the ``unstable`` channel.
+
+.. code:: bash
+
+    $ nix-env -iA nixos.python3Packages.opensensemap-api
+
 Usage
 -----