CONTRIBUTING: Improve and update formatting docs

CONTRIBUTING.md

@@ -531,14 +531,31 @@ If you removed packages or made some major NixOS changes, write about it in the
 
 Names of files and directories should be in lowercase, with dashes between words — not in camel case. For instance, it should be `all-packages.nix`, not `allPackages.nix` or `AllPackages.nix`.
 
+### Formatting
+
+CI [enforces](./.github/workflows/check-nix-format.yml) all Nix files to be
+formatted using the [official Nix formatter](https://github.com/NixOS/nixfmt).
+
+You can ensure this locally using either of these commands:
+```
+nix-shell --run treefmt
+nix develop --command treefmt
+nix fmt
+```
+
+If you're starting your editor in `nix-shell` or `nix develop`,
+you can also set it up to automatically format the file with `treefmt` on save.
+
+If you have any problems with formatting, please ping the
+[formatting team](https://nixos.org/community/teams/formatting/) via
+[@NixOS/nix-formatting](https://github.com/orgs/NixOS/teams/nix-formatting).
+
 ### Syntax
 
 - Set up [editorconfig](https://editorconfig.org/) for your editor, such that [the settings](./.editorconfig) are automatically applied.
 
 - Use `lowerCamelCase` for variable names, not `UpperCamelCase`. Note, this rule does not apply to package attribute names, which instead follow the rules in [package naming](./pkgs/README.md#package-naming).
 
-- New files must be formatted by entering the `nix-shell` from the repository root and running `nixfmt`.
-
 - Functions should list their expected arguments as precisely as possible. That is, write
 
   ```nix