README: Add installation and integration docs

Closes #214

README.md

@@ -14,19 +14,119 @@ The established standard Nix formatting differs considerably from the original o
 
 For more details, see the [RFC implementation tracking issue](https://github.com/NixOS/nixfmt/issues/153).
 
-## Installation
+## Installation And Usage Instructions
 
-- `nixfmt` is in nixpkgs master as of 2019-09-04: 
+### nixpkgs/NixOS
 
-      nix-env -iA nixpkgs.nixfmt
-
-- To get the most recent version, install from master:
-
-      nix-env -f https://github.com/NixOS/nixfmt/archive/master.tar.gz -i
-
-- Nix with flakes
-
-      nix profile install github:NixOS/nixfmt
+`nixfmt` was used as the basis for the official Nix formatter with a standardized formatting.
+The new formatting differs considerably from the original one.
+This is why two packages are in nixpkgs: `nixfmt-classic` with the non-standardized formatting and `nixfmt-rfc-style`.
+
+So installing this `nixfmt` is as simple as adding to the system packages:
+
+```nix
+{ pkgs, ... }:
+
+{
+  environment.systemPackages = [ pkgs.nixfmt-rfc-style ];
+}
+```
+
+### From the repository
+
+It's also possible to install `nixfmt` directly from the repository using `nix-env`.
+This allows the newest changes to be used, which may not be equal to the `nixfmt` version used to format nixpkgs, so this should be done with care!
+Also, updates are not done automatically (as with the system packages).
+
+```
+nix-env -f https://github.com/NixOS/nixfmt/archive/master.tar.gz -i
+```
+
+### nix fmt
+
+[nix fmt](https://nix.dev/manual/nix/latest/command-ref/new-cli/nix3-fmt) (which is a flakes-only feature) can be configured by adding the following to `flake.nix` (assuming a `nixpkgs` input exists):
+```
+{
+  outputs = { nixpkgs, self }: {
+    formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;
+  };
+}
+```
+
+### treefmt
+
+[treefmt](https://github.com/numtide/treefmt) can be used to format repositories consisting of different languages with one command.
+A possible configuration for `nixfmt` in `.treefmt.toml` looks like this:
+```toml
+[formatter.nixfmt-rfc-style]
+command = "nixfmt"
+includes = ["*.nix"]
+```
+
+This only works when `nixfmt-rfc-style` is installed (see above for installation instructions).
+
+`treefmt` can be integrated into text editors and CI to ensure consistent formatting for all filetypes.
+
+### treefmt-nix
+
+[treefmt-nix](https://github.com/numtide/treefmt-nix) automatically configures the correct packages and formatters using a Nix configuration and has native support for `nixfmt`:
+
+```nix
+# ...
+treefmt-nix.mkWrapper nixpkgs {
+  # ... other options ...
+  programs.nixfmt-rfc-style.enable = true;
+}
+```
+
+More information about configuration can be found in [the official README](https://github.com/numtide/treefmt-nix?tab=readme-ov-file#integration-into-nix).
+
+### neovim + nixd
+
+```lua
+local nvim_lsp = require("lspconfig")
+nvim_lsp.nixd.setup({
+   settings = {
+      nixd = {
+         formatting = {
+            command = { "nixfmt" },
+         },
+      },
+   },
+})
+```
+
+This only works when `nixfmt-rfc-style` is installed (see above for installation instructions).
+
+### neovim + nil
+
+```lua
+local nvim_lsp = require("lspconfig")
+nvim_lsp.nil_ls.setup({
+   settings = {
+      ['nil'] = {
+         formatting = {
+            command = { "nixfmt" },
+         },
+      },
+   },
+})
+```
+
+This only works when `nixfmt-rfc-style` is installed (see above for installation instructions).
+
+### neovim + none-ls
+
+```lua
+local null_ls = require("null-ls")
+null_ls.setup({
+    sources = {
+        null_ls.builtins.formatting.nixfmt,
+    },
+})
+```
+
+This only works when `nixfmt-rfc-style` is installed (see above for installation instructions).
 
 ## Development