Migration section added

I have added the attached text to the migration section. It outlines
some of the areas that typically change between major releases. I am
hopeful it will help people in testing migrations.

--
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. +