make README reflect reality

Author: jacobbednarz

README.md

@@ -303,15 +303,9 @@ variables are set (`setenv`) before you do.
 
 ### Clustering support
 
-There are two ways to use the built in clustering support.
-
-- Explicitly set `REDIS_CLUSTER_ENABLED=1` environment variable; and
-- Include a comma delimitered list of hosts in the `REDIS_BACKEND` environment
-  variable.
-
-Either of these approaches will inform the `setBackend` call to expand the
-`REDIS_BACKEND` environment variable and trigger the instantiation of a
-`Credis_Cluster` instead of a `Credis_Client`.
+Clustering support is automatically enabled when the `REDIS_BACKEND` environment
+variable contains a comma separated list of hostnames
+(`10.0.0.1:6379,10.0.0.2:6379`).
 
 ### Logging