add changelog warning about syncproducer config
add changelog warning about syncproducer config

Mitigation for the quasi-breaking change as noted in #816.
remerge/sarama,mailgun/sarama,raisemarketplace/sarama,vrischmann/sarama,raisemarketplace/sarama,vrischmann/sarama,mailgun/sarama,VividCortex/sarama,remerge/sarama,Shopify/sarama,DataDog/sarama,DataDog/sarama,Shopify/sarama,VividCortex/sarama