Add optional auto-failover configuration to example config

The example configuration doesn't mention the auto-failover and having it in there as a commented out block would make taking it into use a simpler task. Something along these lines would work:

# Comment out the following parameters to enable automatic node failover

# <add link here to the auto-failover documentation>

# auto_failover=true

# auto_rejoin=true 

# enforce_simple_topology=true

# replication_user=<username used for replication>

# replication_password=<password used for replication>

If useful, enforce_writable_master=true and enforce_read_only_slaves=true could also be added if they make the failover and cluster management more robust.