Slony for upgrades

Seems we didn't have any mention of Slony for upgrades in the migration
section, so I added it:

<para>
It is also possible to use <productname>Slony</> to create a slave
server with the updated version of <productname>PostgreSQL</>. The
slave can be on the same computer or a different computer. Once it
has synced up with the master server (running the older version of
<productname>PostgreSQL</>), you can switch masters and make the slave
the master and shut down the older database instance. Such a
switch-over results in only several minutes of downtime for an upgrade.
</para>

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://postgres.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +