Revert e5dfc5e8("Move README to markdown")

checkpatch.pl and other scripts fail without README. Revert
the rename for now; we may add README.md as a symlink later.

This reverts commit e5dfc5e8.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-id: 20160720203131.30229-2-bobby.prani@gmail.com
[PMM: tweaked commit message a little]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>