README: Add installation and integration docs

README.md

@@ -14,20 +14,61 @@ 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
+`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`.
 
-- To get the most recent version, install from master:
+So installing this `nixfmt` is as simple as adding to the system packages:
 
-      nix-env -f https://github.com/NixOS/nixfmt/archive/master.tar.gz -i
+```nix
+{ pkgs, ... }:
 
-- Nix with flakes
+{
+  environment.systemPackages = [ pkgs.nixfmt-rfc-style ];
+}
+```
 
-      nix profile install github:NixOS/nixfmt
+### 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
+```
+
+### 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).
 ## Development
 
 ### With Nix