README.OE-Core/README.qemu: Move to markdown format

The READMEs are often viewed from websites so switch to markdown format which is
much as readable as text and much better from html in broswers.

(From OE-Core rev: a9f366a883e4a5dcc364ca2841615c5acc494439)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>