Update README - Add PPX config in Dune file

[gregberns]

I'm new to OCaml and struggled to figure out how to get the PPX working.

Suggestion: In the Using the PPX section, add how to configure the PPX in the dune file.

(libraries pgocaml pgocaml_ppx  )
 (preprocess
  (pps pgocaml_ppx)

Also, if I add try to connect in the main.ml file in bin (not sure if this is important), and open PGOCaml is NOT added, then the EnvVars don't get captured and passed into PGOCaml.connect. It took a bit to figure out why they weren't getting captured during compile.