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

Add a description that kernel config options should be added into a
config file that is placed next to the newly added test.

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>