Add demo shell

shells/grpc.nix

@@ -0,0 +1,14 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  shellHook = ''
+    export DOTFILES_SHELL_GRPC="1"
+  '';
+
+  buildInputs = with pkgs; [
+    grpc      # C based gRPC (C++, Python, Ruby, Obj-C, PHP, C#).
+    grpcui    # Interactive web UI for gRPC, like postman.
+    grpcurl   # Like cURL, but for gRPC servers.
+    protobuf  # gRPC data interchange format.
+    protobufc # C bindings for gRPC.
+  ];
+}