Update CHANGELOG.md for v2.1.1 (#1563)

Author: pranavrth

CHANGELOG.md

@@ -11,6 +11,10 @@ v2.1.1 is a maintenance release with the following fixes and enhancements:
 - Fix some Admin API documentation stating -1 for infinite timeout incorrectly.
   Request timeout can't be infinite.
 
+confluent-kafka-python is based on librdkafka v2.1.1, see the
+[librdkafka release notes](https://github.com/edenhill/librdkafka/releases/tag/v2.1.1)
+for a complete list of changes, enhancements, fixes and upgrade considerations.
+
 
 ## v2.1.0