Fix docs by adding SASL mention

This was previously supported but wasn't documented.

Fix #1990.

Author: jeffwidman

kafka/consumer/group.py

@@ -167,7 +167,8 @@ class KafkaConsumer(six.Iterator):
             message iteration before raising StopIteration (i.e., ending the
             iterator). Default block forever [float('inf')].
         security_protocol (str): Protocol used to communicate with brokers.
-            Valid values are: PLAINTEXT, SSL. Default: PLAINTEXT.
+            Valid values are: PLAINTEXT, SSL, SASL_PLAINTEXT, SASL_SSL.
+            Default: PLAINTEXT.
         ssl_context (ssl.SSLContext): Pre-configured SSLContext for wrapping
             socket connections. If provided, all other ssl_* configurations
             will be ignored. Default: None.