Document Binaryen dependency

Author: vouillon

README.md

@@ -30,9 +30,11 @@ Js_of_ocaml is composed of multiple packages:
 
 ## Requirements
 
-See
+See the
 [opam](https://github.com/ocsigen/js_of_ocaml/blob/master/js_of_ocaml-compiler.opam)
-file for version constraints.
+files for version constraints.
+
+wasm_of_ocaml-compiler additionally depends on a system installation of binaryen (version 119 or later)
 
 ## Supported engines
 

manual/wasm_overview.wiki

@@ -12,6 +12,9 @@ The compiler is provided by the wasm_of_ocaml-compiler package. The <<a_manual c
 The easiest way to install wasm_of_ocaml is to use opam.
 {{{opam install wasm_of_ocaml-compiler js_of_ocaml js_of_ocaml-ppx js_of_ocaml-lwt}}}
 
+Binaryen version 119 or later is required. The opam command above will
+install it automatically if needed.
+
 == Usage ==
 
 Your program must first be compiled using the OCaml bytecode compiler