explicitly require pg_parquet in shared_preload_libraries

[aykut-bozkurt]

• Explicitly require pg_parquet in shared_preload_libraries because we hook into Postgres utility hook,
• Improve README.md about installation.
  • specify --features pg<version> if the pg version is not the latest supported (default) by pg_parquet.
  • specify cargo-pgrx version as the same as pgrx dependency in Cargo.toml.

Closes #68.
Closes #149.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.89%. Comparing base (8cec92c) to head (3eee28c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
- Coverage   90.90%   90.89%   -0.01%     
==========================================
  Files          92       92              
  Lines       10420    10423       +3     
==========================================
+ Hits         9472     9474       +2     
- Misses        948      949       +1     

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[marcoslot]

We sometimes also put an IsBinaryUpgrade check here to avoid getting unintentionally loaded during pg_upgrade