direnv: provide example config, update Readme

The recommended direnv config is now shipped as an example file,
allowing to just copy it for using. Readme is updated accordingly.

The recommended NixOS config for direnv now uses the recommended nix-community use_flake
hook as well.

.envrc.example

@@ -0,0 +1,2 @@
+use flake . --impure --allow-dirty --no-write-lock-file
+build_channels_dir

README.md

@@ -39,11 +39,15 @@ Automatically enter the dev shell with direnv
 
 Use `direnv` to automatically enter the dev shell when you change to the fc-nixos directory.
 
+A recommended direnv config is shipped in `.envrc.example` to use it in your repo checkout just
+`cp .envrc{.example,} && direnv allow`.
+
+We recommend the usage of the [nix-community/nix-direnv](https://github.com/nix-community/nix-direnv) hook
+instead of the one shipped by direnv itself.
 To set it up with `home-manager`, see:
 https://github.com/nix-community/nix-direnv?tab=readme-ov-file#via-home-manager
 
-Without home-manager
---------------------
+### Without home-manager
 
 On a NixOS machine, enabling `programs.direnv.enable` should be enough.
 
@@ -54,18 +58,14 @@ Add `/etc/local/nixos/dev_vm.nix`, for example:
       nix.extraOptions = ''
         keep-outputs = true
       '';
-      programs.direnv.enable = true;
+      programs.direnv = {
+        enable = true;
+        nix-direnv.enable = true;
+      };
     }
 
 Rebuild the system, close the shell/tmux session and log in again.
 
-In `fc-nixos`, add an `.envrc` file like:
-
-    use flake . --impure --allow-dirty
-    build_channels_dir
-
-Then, run `direnv allow` to build and enter the dev shell.
-
 Run `direnv allow` again if the dev shell disappears or doesn't reload automatically.