Added guide for upgrading Postgres using Slony
What does this MR do?
This adds a guide for upgrading PostgreSQL using Slony.
Are there points in the code the reviewer needs to double check?
Spelling, grammar, etc
Why was this MR needed?
This process is not documented and we'll need to provide steps on how to do this before we start shipping PostgreSQL 9.6 with omnibus-gitlab.
Does this MR meet the acceptance criteria?
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CHANGELOG entry added -
Documentation created/updated -
Conform by the style guides -
Branch has no merge conflicts with master(if you do - rebase it please) -
Squashed related commits together