add hint for installation on existing postgres

[resamaraschi]

No description provided.

[aykut-bozkurt]

cargo pgrx init --pg17 $(which pg_config) (you can configure other pg versions too, e.g. cargo pgrx init --pg16 "/usr/bin/postgres/16/bin/pg_config" --pg17 "/usr/bin/postgres/17/bin/pg_config") before cargo pgrx install or cargo pgrx run is required.

--manifest_path and --pg_config should not be required during cargo pgrx install. We might still document cargo pgrx install if all you want is to install the extension without connecting to a session. (cargo pgrx run also installs the extension but additionally initializes a data directory and connects to a session there)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.84%. Comparing base (5766819) to head (b3e0301).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #119   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files          83       83           
  Lines       10494    10494           
=======================================
  Hits         9638     9638           
  Misses        856      856           

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