Add development setup instructions to README

[SeanTAllen]

The README documents how to install and use the library, but there's no documentation for contributors on how to build from source and run tests. Key information that should be documented:

• Makefile targets (make, make unit-tests, make integration-tests, make build-examples)
• Integration tests require a running PostgreSQL 14.5 instance (make start-pg-container / make stop-pg-container)
• SSL build flag requirement
• Environment variables for test configuration (POSTGRES_HOST, POSTGRES_PORT, POSTGRES_USERNAME, POSTGRES_PASSWORD, POSTGRES_DATABASE)

A "Development" section in the README would be the natural place — the CONTRIBUTING.md is org-wide boilerplate and shouldn't hold project-specific instructions.

[SeanTAllen]

We also need to note that the test certs in assets cant be world readable otherwise integration tests will fail.

[SeanTAllen]

Note for when this is tackled: PR #82 changed the container setup. make start-pg-containers (plural) now starts two PostgreSQL containers — one plain on port 5432 and one with SSL enabled on port 5433. There's also a POSTGRES_SSL_HOST and POSTGRES_SSL_PORT env var for the SSL container. The CI setup uses a pre-built GHCR image (ghcr.io/ponylang/postgres-ci-pg-ssl:latest) for the SSL container rather than the local volume-mount approach, with a build-ci-image.yml workflow to rebuild it.