Documentation: KUnit: add note about mrproper in start.rst

The "Getting Started" guide should be beginner-friendly, therefore
add a note about the requirement of a clean source tree when running
kunit_tool for the first time, and its related error.

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>