Windows instructions for OpenJDK9

[Patrick Reinhart]

Hi Magnus,

> Some comments on the Windows parts:
> 
> The officially supported build environment for Windows is Cygwin. I have not heard about babun before, but if it integrates Cygwin at its core, it should probably work. I'm a bit wary about the existence of "c:/..." style paths. Internally, the build system uses only /cygdrive/c/... style paths, and if a c:/... path slips by, things are likely to break.

I changed that part i the documentation already to use constantly /cygdrive/c

> It is recommended to use the mercurial from cygwin instead of TortoiseHG, to avoid unpleasant surprises related to file permissions, and once again, the c:/... paths problem. You can use TortoiseHG as a shell add-on in Windows, but don't use it for cloning.

This is already changed to the cygwin’s mercurial package as it’s needed for the webrev script to work correctly

> get_source.sh is not executable, so you need to run „bash get_source.sh“, not "./get_source.sh".

Also changed in the documentation

> On a more general note, I have noted that there is a widespread phenomenon on creating Yet Another Guide To Building OpenJDK, most of them more or less degraded permutations on the best practices. I think it would be better if there was a single official document that was good enough for everyone to use. I realize that the current official build documentation is severly lacking in usability, but I'm not sure what to do about it, or where to put a suitable document. I'm afraid part of the problem is that it is not clear where to go for proper, up-to-date, documentation.
> 
> /Magnus

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.

Patrick