clan-config: add readme

Also add capability of reading an option value

docs/clan-config.md

@@ -0,0 +1,65 @@
+# cLAN config
+
+`clan config` allows you to manage your nixos configuration via the terminal.
+Similar as how `git config` reads and sets git options, `clan config` does the same with your nixos options
+It also supports auto completion making it easy to find the right options.
+
+## Set up clan-config
+
+Add the clan tool to your flake inputs:
+
+```
+clan.url = "git+https://git.clan.lol/clan/clan-core";
+```
+
+and inside the mkFlake:
+```
+imports = [
+  inputs.clan.flakeModules.clan-config
+];
+```
+
+Add an empty config file and add it to git
+```command
+echo "{}" > ./clan-settings.json
+git add ./clan-settings.json
+```
+
+Import the clan-config module into your nixos configuration:
+```nix
+{
+  imports = [
+    # clan-settings.json is located in the same directory as your flake.
+    # Adapt the path if necessary.
+    (builtins.fromJSON (builtins.readFile ./clan-settings.json))
+  ];
+}
+
+
+```
+
+Make sure your nixos configuration is set a default
+```nix
+{self, ...}: {
+  flake.nixosConfigurations.default = self.nixosConfigurations.my-machine;
+}
+```
+
+Use all inputs provided by the clan-config devShell in your own devShell:
+
+```nix
+{ ... }: {
+  perSystem = { pkgs, self', ... }: {
+    devShells.default = pkgs.mkShell {
+      inputsFrom = [ self'.devShells.clan-config ];
+      # ...
+    };
+  };
+}
+```
+
+re-load your dev-shell to make the clan tool available.
+
+```command
+clan config --help
+```