add instructions on how to install dune

Author: zapashcanon

interpreter/README.md

@@ -17,6 +17,8 @@ The text format defines modules in S-expression syntax. Moreover, it is generali
 
 You'll need OCaml 4.12 or higher. Instructions for installing a recent version of OCaml on multiple platforms are available [here](https://ocaml.org/docs/install.html). On most platforms, the recommended way is through [OPAM](https://ocaml.org/docs/install.html#OPAM).
 
+You'll also need to install the dune build system. See the [installation instructions](https://github.com/ocaml/dune#installation-1).
+
 Once you have OCaml, simply do
 
 ```