Update build documentation

Because stack builds in parallel, elm-reactor could conceivably be
built before elm-make, causing the overall build to fail. Thus
the user has to build them in sequential invocations of stack for
everything to work. A built script might be appropriate.

versions/README.md

@@ -1,4 +1,4 @@
-# Experimental Build System
+# Experimental build system
 
 ## Getting the latest version of stack
 
@@ -14,9 +14,15 @@ of accomplishing this is to first:
 ## Installing Elm
 
 1. Download the desired `stack.yaml` into an empty directory:
-    curl https://raw.githubusercontent.com/elm-lang/elm-platform/master/versions/0.15.1/stack.yaml > stack.yaml
-2. Run `stack install` to install elm-platform into `~/.local/bin` or its
-   equivalent location.
+   ```
+   curl https://raw.githubusercontent.com/elm-lang/elm-platform/master/versions/0.15.1/stack.yaml > stack.yaml
+   ```
+2. Run the following:
+   ```
+   stack build elm-compiler elm-package elm-make elm-repl && stack build elm-reactor
+   ```
+   This will install the Elm platform into `~/.local/bin` or its equivalent
+   location on your OS.
 3. Optionally delete the directory when done; stack does all its work in a
    subdirectory named `.stack-work`.