added pre-commit, direnv with shell.nix

.envrc

@@ -0,0 +1 @@
+use nix

.gitignore

@@ -0,0 +1 @@
+.direnv

.pre-commit-config.yaml

@@ -0,0 +1,5 @@
+repos:
+  - repo: https://github.com/markdownlint/markdownlint
+    rev: v0.13.0
+    hooks:
+      - id: markdownlint_docker

shell.nix

@@ -0,0 +1,7 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+ pkgs.mkShell {
+   packages = with pkgs; [
+      pre-commit
+   ];
+}