README: add maintainers section for hardware reference boards

README: add maintainers section for hardware reference boards

Kevin Hao has stepped up to take care of issues specific to the meta-yocto-bsp
hardware reference boards. I'll also serve as a maintaier, so adding my name
as well.

To make this clear, let's add it to the README.hardware file that describes
the boards.

cc: Kevin Hao <kexin.hao@windriver.com>
(From meta-yocto rev: 7ea7fdf4a9ad0fcedf9a939e20146342d6ab1c57)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>