add nix-shell boilerplate

This enables users of nix-shell and lorri to automagically have the
correct development environment by simply changing directory into a
checkout of this repo. For more information on this see the following
links:

- https://christine.website/blog/how-i-start-nix-2020-03-08
- https://direnv.net/
- https://www.tweag.io/blog/2019-03-28-introducing-lorri/

Signed-off-by: Christine Dodrill <me@christine.website>

.envrc

@@ -0,0 +1 @@
+eval "$(lorri direnv)"

shell.nix

@@ -0,0 +1,10 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    go goimports gopls
+
+    # keep this line if you use bash
+    pkgs.bashInteractive
+  ];
+}