README.md: Add contributing section
README.md: Add contributing section

This also fixes a check in the yocto project compatible script.
https://bugzilla.yoctoproject.org/show_bug.cgi?id=11131

Signed-off-by: Andrei Gherzan <0401b4310c02ae595fc6a2b84ead073cab9b0442@huawei.com>

agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi