Document that Reason is supported out of the box

doc/reference/files/dune-project/dialect.rst

@@ -65,3 +65,9 @@ dialect
       This field supports the same sub-fields as ``implementation``.
 
       .. versionchanged:: 3.9 This field is made optional.
+
+Dune comes with some dialects pre-set and enabled. The most common one is
+`ocaml` which consumes `.ml` and `.mli` files and uses `ocamlformat` for
+formatting. There also exists `reason` which can be used in Dune projects
+without further configuration and uses `.re`/`.rei` files. For formatting
+`refmt` is used.