add examples

examples/hello_world.nix

@@ -0,0 +1,30 @@
+let
+  pkgs = import <nixpkgs> { overlays = [ (import ../pkgs) ]; };
+in {
+  bash = pkgs.writeBash "hello-world" ''
+    echo 'hello world'
+  '';
+  c = pkgs.writeC "hello-world"  {} ''
+    #include <stdio.h>
+    int main() {
+      printf("hello world\n");
+      return 0;
+    }
+  '';
+  dash = pkgs.writeDash "hello-world" ''
+    echo 'hello world'
+  '';
+  haskell = pkgs.writeHaskell "hello-world" [] ''
+    main = do
+      putStrLn "hello world"
+  '';
+  python2 = pkgs.writePython2 "hello-world" {} ''
+    print "hello world"
+  '';
+  python3 = pkgs.writePython3 "hello-world" {} ''
+    print("hello world")
+  '';
+  sed = pkgs.writeDash "sed-example" ''
+    echo xxx | ${pkgs.writeSed "hello-world" "s/xxx/hello world/"}
+  '';
+}