add shell.nix with correct build environment

Signed-off-by: Jakub Sokołowski <jakub@status.im>

shell.nix

@@ -0,0 +1,10 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  name = "discover";
+  buildInputs = with pkgs; [
+    nodejs-12_x
+    (yarn.override { nodejs = nodejs-12_x; })
+    python2
+  ];
+}