Add Postgres setup instructions in README

[raywan]

I spent some time creating the Postgres tables until I found that there exists a setup script. I don't think this is documented anywhere.

It would be nice if the DB and tables were setup upon initialization though, as referenced in #96.

[codecov-io]

Codecov Report

Merging #119 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #119   +/-   ##
=========================================
  Coverage     63.03%   63.03%           
  Complexity      271      271           
=========================================
  Files            66       66           
  Lines          1339     1339           
  Branches        136      136           
=========================================
  Hits            844      844           
  Misses          466      466           
  Partials         29       29

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