Add warning about CCM updateconf configuration parameters overloading to README.md
patch by Ekaterina Dimitrova; revewed by Berenguer Blasi for
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
+
+
+CCM (Cassandra Cluster Manager)
+====================================================
+
A script/library to create, launch and remove an Apache Cassandra cluster on
localhost.
