Locking does not work with clustered MySQL setups

[lhz]

https://www.percona.com/doc/percona-xtradb-cluster/5.5/limitation.html
https://mariadb.com/kb/en/mariadb/mariadb-galera-cluster-known-limitations/

It would be a good idea to mention this in the README, we just found out the hard way.

[mceachen]

updated the readme.