[PATCH v1] sync doc/README.socfpga with new info

[Sune Brian]

> Right, so the first thing to do is migrate doc/README.socfpga to rST
> probably as doc/board/socfpga/index.rst (and include that in
> doc/board/index.rst). Then work in some improvements, thanks.

I can only work to this stage. Other migration should done by
Intel stuffs. As I modified to location and the execution of the
handoff tool I have obligation to tell user how and where.
But other than that the rst should pickup back by Intel-Altera.

Brian