CONTRIBUTING.md: Mention nixfmt instead of manual formatting rules

CONTRIBUTING.md

@@ -561,132 +561,7 @@ Names of files and directories should be in lowercase, with dashes between words
 
 - 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).
 
-- Function calls with attribute set arguments are written as
-
-  ```nix
-  foo {
-    arg = <...>;
-  }
-  ```
-
-  not
-
-  ```nix
-  foo
-  {
-    arg = <...>;
-  }
-  ```
-
-  Also fine is
-
-  ```nix
-  foo { arg = <...>; }
-  ```
-
-  if it's a short call.
-
-- In attribute sets or lists that span multiple lines, the attribute names or list elements should be aligned:
-
-  ```nix
-  {
-    # A long list.
-    list = [
-      elem1
-      elem2
-      elem3
-    ];
-
-    # A long attribute set.
-    attrs = {
-      attr1 = short_expr;
-      attr2 =
-        if true then big_expr else big_expr;
-    };
-
-    # Combined
-    listOfAttrs = [
-      {
-        attr1 = 3;
-        attr2 = "fff";
-      }
-      {
-        attr1 = 5;
-        attr2 = "ggg";
-      }
-    ];
-  }
-  ```
-
-- Short lists or attribute sets can be written on one line:
-
-  ```nix
-  {
-    # A short list.
-    list = [ elem1 elem2 elem3 ];
-
-    # A short set.
-    attrs = { x = 1280; y = 1024; };
-  }
-  ```
-
-- Breaking in the middle of a function argument can give hard-to-read code, like
-
-  ```nix
-  someFunction { x = 1280;
-    y = 1024; } otherArg
-    yetAnotherArg
-  ```
-
-  (especially if the argument is very large, spanning multiple lines).
-
-  Better:
-
-  ```nix
-  someFunction
-    { x = 1280; y = 1024; }
-    otherArg
-    yetAnotherArg
-  ```
-
-  or
-
-  ```nix
-  let res = { x = 1280; y = 1024; };
-  in someFunction res otherArg yetAnotherArg
-  ```
-
-- The bodies of functions, asserts, and withs are not indented to prevent a lot of superfluous indentation levels, i.e.
-
-  ```nix
-  { arg1, arg2 }:
-  assert system == "i686-linux";
-  stdenv.mkDerivation { /* ... */ }
-  ```
-
-  not
-
-  ```nix
-  { arg1, arg2 }:
-    assert system == "i686-linux";
-      stdenv.mkDerivation { /* ... */ }
-  ```
-
-- Function formal arguments are written as:
-
-  ```nix
-  { arg1, arg2, arg3 }: { /* ... */ }
-  ```
-
-  but if they don't fit on one line they're written as:
-
-  ```nix
-  { arg1, arg2, arg3
-  , arg4
-  # Some comment...
-  ,  argN
-  }: { }
-  ```
+- 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