doc: dev-tools: kselftest.rst: update contributing new tests

Add a description that the kernel headers should be used as far as it is
possible and then the system headers.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>