README: add section on testing a board
README: add section on testing a board

This adds a section on testing a board before opening a Pull Request and the
reasons behind these requirements.

Signed-off-by: Quentin Schulz <222e928ffea5b2038b4ed3e727cfc6ce1f518a39@free-electrons.com>

kernelci/lava-ci-staging,kernelci/lava-ci-staging,kernelci/lava-ci-staging