update README re producer futures
update README re producer futures

Signed-off-by: Yung-Chin Oei <e4c1ab9da74060df152545062ee274845648ed6c@yungchin.nl>
(cherry picked from commit a63797cfa7307a0d7665304da5181e3c3b60f2fc)
Signed-off-by: Yung-Chin Oei <e4c1ab9da74060df152545062ee274845648ed6c@yungchin.nl>

wikimedia/operations-debs-python-pykafka,benauthor/pykafka,thedrow/samsa,wikimedia/operations-debs-python-pykafka,benauthor/pykafka,thedrow/samsa,wikimedia/operations-debs-python-pykafka,benauthor/pykafka,thedrow/samsa