Add warning about CCM updateconf configuration parameters loading

README.md

@@ -1,6 +1,27 @@
 CCM (Cassandra Cluster Manager)
 ====================================================
 
+WARNING - CCM configuration changes using updateconf does not happen according to CASSANDRA-17379
+-------------------------------------------------------------------------------------------------
+
+After CASSANDRA-15234, to support the Python upgrade tests CCM updateconf is replacing new key name and value in case 
+the old key name and value is provided.  
+For example, if you add to config permissions_validity_in_ms, it will replace permissions_validity in default cassandra.yaml
+This was needed to ensure correct overloading as CCM cassandra.yaml has keys sorted lexicographically.
+CASSANDRA-17379 was opened to improve the user experience and deprecate the overloading of parameters in cassandra.yaml.
+In CASSANDRA 4.1+, by default, we refuse starting Cassandra with a config containing both old and new config keys for the
+same parameter. Start Cassandra with 
+`-Dcassandra.allow_new_old_config_keys=true` to override. For historical reasons duplicate config keys in cassandra.yaml 
+are allowed by default, start Cassandra with `-Dcassandra.allow_duplicate_config_keys=false` to disallow this. Please note
+that key_cache_save_period, row_cache_save_period, counter_cache_save_period will be affected only by 
+`-Dcassandra.allow_duplicate_config_keys`. Ticket CASSANDRA-17949 was opened to decide the future of CCM updateconf
+post CASSANDRA-17379, until then - bear in mind that old replace new parameters' in cassandra.yaml when using updateconf 
+even if `-Dcassandra.allow_new_old_config_keys=false` is set by default. 
+TLDR Do not exercise overloading of parameters in CCM if possible. Also, the mentioned changes are done only in master branch.
+Probably the best way to handle cassandra 4.1 in CCM at this point is to set `-Dcassandra.allow_new_old_config_keys=false` and 
+`-Dcassandra.allow_duplicate_config_keys=false` to prohibit any kind of overloading when using CCM master and CCM released 
+versions
+
 A script/library to create, launch and remove an Apache Cassandra cluster on
 localhost.