Add explicit system dependencies, nice post message

Author: fxfactorial

opam/opam

@@ -29,4 +29,18 @@ depends: [
   "yojson"
 ]
 
-available: [ ocaml-version >= "4.02.3" ]
+available: [ ocaml-version >= "4.02.3" ]
+
+depexts: [
+  [["debian"] ["nodejs"]]
+  [["homebrew" "osx"] ["nodejs"]]
+  [["ubuntu"] ["nodejs"]]
+]
+
+post-messages: [
+  "Now you can write OCaml and execute on node."
+  "Example assuming file name of c.ml:"
+  "ocamlfind ocamlc c.ml -linkpkg -package nodejs -o T.out"
+  "js_of_ocaml T.out"
+  "node T.js"
+]