title: How does KSQL read(consume msgs) data from a topic, already containing some messages
body: <p>I have created a stream from Kafka using KSQL for a Kafka topic which already contains some messages. But not receiving any msg from that topic into stream created. 
  Messages are in Avro format and generated after some interval. </p>
  
  <p>I want to read from starting i.e. from earliest msg. Also tried setting offset property to earliest but not receiving any msg.</p>
  
  <p><code>create stream sample_transition with(topic_name='transition',value_format='avro');</code></p>