Configuration/Cluster Set Up chapter miss information about ERL_EPMD_ADDRESS env variable

[sblaisot]

When setting-up couchdb cluster following documentation at https://docs.couchdb.org/en/stable/setup/cluster.html there is no indication for the need to set the ERL_EPMD_ADDRESS environment variable to expose epmd to other nodes, in particular when using the couchdb provided debian package where this variable is defined in /etc/default/couchdb to 127.0.0.1

I think this should be added to the documentation to avoid loosing time when configuring a cluster.