Better README.md for dlang.org

[Andrei Alexandrescu via Digitalmars-d]

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.


Andrei