Better README.md for dlang.org

[Andrej Mitrovic via Digitalmars-d]

On 1/20/15, Andrei Alexandrescu via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> https://github.com/D-Programming-Language/dlang.org/blob/master/README.md
>
> 1. If instructions don't work for you, reply here.
>
> 2. Additions for Windows would be welcome. FWIW I think the Windows
> makefile suffers from a bit of bitrot.

Btw, apparently we can use a contributing file that automatically
shows up when people make pull requests:

https://help.github.com/articles/setting-guidelines-for-repository-contributors/