Adding an example for replication configuration to pg_hba.conf

I reviewed the process for configuring replication, and found that we
don't have an example for allowing replication access in pg_hba.conf.
Before we release 9.1, I think we should add this example to make it
more obvious this is a necessary part of replication configuration.

At the risk of starting an epic bikeshedding thread, I've attached a
small patch that includes an example for both ipv4 and ipv6 localhost
configuration.

-selena

-- 
http://chesnok.com