README: Add a contributing section
README: Add a contributing section

This section explains how to send patches to the Yocto Project mailing list
and the required patches format

Signed-off-by: Romain Perier <b3f268131c5a6ec066690d62d0a6c5c2b0bbcd85@gmail.com>

hizukiayaka/meta-rockchip,hizukiayaka/meta-rockchip,hizukiayaka/meta-rockchip,twoerner/meta-rockchip-twoerner,twoerner/meta-rockchip-twoerner