Add comment for initialPositionInStreamExtended

Author: Ava Huang

samples/sample.properties

@@ -24,7 +24,10 @@ processingLanguage = python/2.7
 # Valid options at TRIM_HORIZON or LATEST.
 # See http://docs.aws.amazon.com/kinesis/latest/APIReference/API_GetShardIterator.html#API_GetShardIterator_RequestSyntax
 initialPositionInStream = TRIM_HORIZON
-# See 'Timestamp' under http://docs.aws.amazon.com/kinesis/latest/APIReference/API_GetShardIterator.html#API_GetShardIterator_RequestSyntax
+
+# To specify an initial timestamp from which to start processing records, please specify timestamp value for 'initiatPositionInStreamExtended',
+# and uncomment below line with right timestamp value.
+# See more from 'Timestamp' under http://docs.aws.amazon.com/kinesis/latest/APIReference/API_GetShardIterator.html#API_GetShardIterator_RequestSyntax
 #initialPositionInStreamExtended = 1636609142
 
 # The following properties are also available for configuring the KCL Worker that is created