Documentation: kunit: add documentation for kunit_tool
Add documentation for the Python script used to build, run, and collect
results from the kernel known as kunit_tool. kunit_tool
(tools/testing/kunit/kunit.py) was already added in previous commits.
Signed-off-by: Brendan Higgins <brendanhiggins@google.com> Reviewed-by: David Gow <davidgow@google.com> Cc: Randy Dunlap <rdunlap@infradead.org> Acked-by: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>