Document that Reason/Rescript is supported out of the box

Signed-off-by: Marek Kubica <marek@tarides.com>

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

@@ -65,3 +65,20 @@ dialect
       This field supports the same sub-fields as ``implementation``.
 
       .. versionchanged:: 3.9 This field is made optional.
+
+
+Default dialects
+----------------
+
+In the default configuration Dune comes with two dialects pre-set and enabled:
+
+* `ocaml` for the default OCaml syntax which consumes `.ml` and `.mli` files
+  and uses `ocamlformat` for formatting.
+* `reason` for the Reason syntax and enabled in `.re`/`.rei` files. For
+  formatting `refmt` is used.
+
+When `(using melange)` is enabled in the `dune-project` a third dialect is
+enabled:
+
+* `rescript` for the ReScript syntax which is used in `.res` and `.resi` files.
+  The formatter in this case is `rescript_syntax`.