Added a jekyll.nix shell

jekyll.nix

@@ -0,0 +1,8 @@
+let pkgs = import (fetchTarball "channel:nixpkgs-unstable") { };
+in pkgs.mkShell {
+  buildInputs = [ pkgs.ruby pkgs.r-gems pkgs.jekyll ];
+  shellHook = ''
+    exec fish
+    exec gem install bundler
+  '';
+}