elm: add documentation on how to use update scripts and run them properly

pkgs/development/compilers/elm/default.nix

@@ -1,5 +1,11 @@
 { lib, stdenv, buildEnv, haskell, nodejs, fetchurl, fetchpatch, makeWrapper }:
 
+# To update:
+# 1) Update versions in ./update-elm.rb and run it.
+# 2) Checkout elm-reactor and run `elm-package install -y` inside.
+# 3) Run ./elm2nix.rb in elm-reactor's directory.
+# 4) Move the resulting 'package.nix' to 'packages/elm-reactor-elm.nix'.
+
 let
   makeElmStuff = deps:
     let json = builtins.toJSON (lib.mapAttrs (name: info: info.version) deps);
@@ -61,9 +67,6 @@ let
       in elmPkgs // {
         inherit elmPkgs;
         elmVersion = elmRelease.version;
-
-        # To unbreak elm-compiler
-        language-ecmascript = self.language-ecmascript_0_17_0_2;
       };
   };
 in hsPkgs.elmPkgs // {