Write announcement post for Postgres change

Created by: efritz

There is a large-ish change coming to the distribution of Sourcegraph 3.21+: The introduction of a second database. I think it could be a good opportunity for a blog post to both document the change in architecture, our reasons for doing so, and instructions on what to do if you are already heavily involved with our own primary database (of course this is not a substitute for actual distribution instructions).