add SslConfig notes to README
add SslConfig notes to README

Signed-off-by: Yung-Chin Oei <e4c1ab9da74060df152545062ee274845648ed6c@yungchin.nl>

wikimedia/operations-debs-python-pykafka,wikimedia/operations-debs-python-pykafka,wikimedia/operations-debs-python-pykafka