Add NixOS install instruction

Author: fabaff

README.rst

@@ -21,6 +21,13 @@ On a Fedora-based system or on a CentOS/RHEL machine which has EPEL enabled.
 
     $ sudo dnf -y install python3-netdata
 
+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.python39Packages.netdata
+
 Usage
 -----