update README kafka version badge to include 4.0

Author: dpkp

README.rst

@@ -1,7 +1,7 @@
 Kafka Python client
 ------------------------
 
-.. image:: https://img.shields.io/badge/kafka-3.9--0.8-brightgreen.svg
+.. image:: https://img.shields.io/badge/kafka-4.0--0.8-brightgreen.svg
     :target: https://kafka-python.readthedocs.io/en/master/compatibility.html
 .. image:: https://img.shields.io/pypi/pyversions/kafka-python.svg
     :target: https://pypi.python.org/pypi/kafka-python