nix: add mdsh to the devenv

README.md

@@ -38,6 +38,7 @@ quite tricky.
 ## Usage
 
 `$ treefmt --help`
+
 ```
 treefmt 0.3.0
 ✨  format all your language!

default.nix

@@ -28,6 +28,7 @@ let
       go
       haskellPackages.cabal-fmt
       haskellPackages.ormolu
+      mdsh
       nixpkgs-fmt
       nodePackages.prettier
       python3.pkgs.black
@@ -40,6 +41,11 @@ let
       mdbook
     ];
 
+    shellHook = ''
+      # Put the treefmt binary on the PATH when it's built
+      export PATH=$PWD/target/debug:$PATH
+    '';
+
     buildInputs = with nixpkgs; lib.optionals stdenv.isDarwin [ darwin.apple_sdk.frameworks.Security libiconv ];
 
     doCheck = true;