README: add "Contributing" guideline

The "Contributing" guideline section should help people sending their
patches correctly.

A similar section is available in nearly all
layers/repositories hosted on yoctoproject.org. Therefore add it also
the the psplash repository.

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>