More links to gdc, ldc, emphasize download

I'm not sure whether the dlang.org CONTRIBUTING file is the most fitting location for the link. There is also http://wiki.dlang.org/Get_involved, but it equally seems to be a bit lacking. Maybe we should (probably after this is merged) work on a new page that explains that there are three main repositories, the website/tools, and GDC/LDC, and so on to provide some orientation.