Windows instructions for OpenJDK9

[Patrick Reinhart]

I will try to update the windows part for cygwin

> +1 to keeping the core build README up to date - there should be no reason why we can't start submitting web revs to improve it...
> 
> The documentation Wiki page was created due the need to have a more quick way of keeping the documentation up-to-date. I personally would prefer to have the documentation closer to the OpenJDK repositories itself as the README-builds.html.
> 
> 
> -- 
> Cheers,
> Martijn

Patrick