OSX package installation process with Drag&Drop

[Henri Gomez]

> As a prerequisite to using OpenJDK, we'd ask that you install the Apple Java Developer Preview at:
> <https://connect.apple.com/cgi-bin/WebObjects/MemberSite.woa/wa/getSoftware?bundleID=20920>
>
> This preview will ensure that you have a /Library/Java/JavaVirtualMachines directory to land the 1.7.0.jdk into, as well as the new API that OpenJDK is now relying on.

I added this mention in main page of openjdk-osx-build site and in
NewInstallationProcess wiki page.