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@momjian.us>        http://momjian.us
  EnterpriseDB                             http://postgres.enterprisedb.com

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