readme: document Qemu port.

[errordeveloper]

No description provided.

[pfalcon]

Thanks for this patch, but I thought it would be structure a bit different. The top-level README is already pretty big, and really should not grow any more, especially with things not interesting to everyone. Some ports (e.g. windows/) started to add per-port READMEs, and that's how I envisioned it for qemu. I'm also not sure about formatting changes to top README - it should remain to be plain-text readable, and I'm not sure too much markup and non-ascii chars help with that.

Summing up, I would vote for just extracting qemu port description as qemu/README and avoid other changes. I can go for that if there're no objections.

[dpgeorge]

I agree with @pfalcon's points, especially that the main README (in the root dir) should be as concise as possible.

[pfalcon]

Thanks for splitting out qemu-arm/README.md, that was merged. I didn't merge the rest, feel free to rebase and resubmit with more advocacy why these changes are needed. But I already commented on formatting changes, and single textual change makes it just more confusing with "both architectures" (if this was a wikipedia edit, it would get "[which?]" mark) and "probably".