Update rescript.md

Author: cannorin

docs/rescript.md

@@ -1,3 +1,7 @@
+> **Note:** ReScript support is currently available in `2.0.0-beta.0` or later.
+> 
+> Please install it with `npm install -g @ocsigen/ts2ocaml@beta`.
+
 # ts2ocaml for ReScript
 
 Generates binding for ReScript.
@@ -837,4 +841,4 @@ switch x->FooBar.box {
   | Foo(foo) => ...
   | Bar(bar) => ...
 }
-```
+```