Restore shell.nix

I believe `use_nix` looks for shell.nix and then default.nix files, so I was
wrong earlier when I said that I could get rid of shell.nix files altogether.

I need to learn more about `use_nix` and getting environments from default.nix
files.

zoo/shell.nix

@@ -0,0 +1,10 @@
+let
+  briefcase = import /home/wpcarro/briefcase {};
+in briefcase.buildHaskell.shell {
+  deps = hpkgs: with hpkgs; [
+    servant-server
+    aeson
+    warp
+    rio
+  ];
+}