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>