nixd/docs/configuration: add Emacs configuration (#612)

nixd/docs/configuration.md

@@ -110,6 +110,33 @@ nvim_lsp.nixd.setup({
 ```
 </details>
 
+<details>
+  <summary>Emacs</summary>
+
+  Configuration via [lsp-mode](https://github.com/emacs-lsp/lsp-mode) plugin. As of Oct 24, 2024, lsp-mode master has support for nixd autocompletion and formatting options. 
+
+  ```elisp
+(use-package nix-mode
+  :after lsp-mode
+  :ensure t
+  :hook
+  (nix-mode . lsp-deferred) ;; So that envrc mode will work
+  :custom
+  (lsp-disabled-clients '((nix-mode . nix-nil))) ;; Disable nil so that nixd will be used as lsp-server
+  :config
+  (setq lsp-nix-nixd-server-path "nixd"
+        lsp-nix-nixd-formatting-command [ "nixfmt" ]
+        lsp-nix-nixd-nixpkgs-expr "import <nixpkgs> { }"
+        lsp-nix-nixd-nixos-options-expr "(builtins.getFlake \"/home/nb/nixos\").nixosConfigurations.mnd.options"
+        lsp-nix-nixd-home-manager-options-expr "(builtins.getFlake \"/home/nb/nixos\").homeConfigurations.\"nb@mnd\".options"))
+
+(add-hook! 'nix-mode-hook
+           ;; enable autocompletion with company
+           (setq company-idle-delay 0.1))
+
+  ```
+</details>
+
 ### Configuration overview
 
 > [!NOTE]