add krops trick about channels

content/nixos/krops.md

@@ -456,3 +456,45 @@ Now you can just have to call the `nix-prefetch-git` command
 and the commit reference will be updated, and is fixed.
 
 This should also make it simpler to maintain different channels on different machines.
+
+## Use Packages from other channels
+
+It is very easy to install packages from different channels.
+
+For example add `nixpkgs-unstable` the same way you add `nixpkgs`.
+
+```nix
+  source =  lib.evalSource [
+    {
+      nixpkgs.git = {
+        ref = "nixos-18.09";
+        url = https://github.com/NixOS/nixpkgs-channels;
+      };
+      nixpkgs-unstable.git = {
+        ref = "nixos-unstable";
+        url = https://github.com/NixOS/nixpkgs-channels;
+      };
+      nixos-config.file = toString ./configuration.nix;
+    }
+  ];
+```
+
+To install a package from the `unstable` channel you just have to import the channel
+and call the packages from there.
+
+```nix
+{ config, pkgs, ... }:
+let
+  unstable = import <nixpkgs-unstable> {};
+in {
+  environment.systemPackages = [
+
+    # install gimp from stable channel
+    pkgs.gimp
+
+    # install inkscape from unstable channel
+    unstable.inkscape
+
+  ];
+}
+```