Update README.md to mention PGDATA changes for Pg18 images

[phillipross]

Comment from PR #430 suggested documenting PG18 + PGDATA changes:

• https://github.com/docker-library/docs/tree/master/postgres#pgdata
• Change PGDATA in 18+ to /var/lib/postgresql/MAJOR/docker postgres#1259

[tianon]

Can you please be more specific? We did document the changes: 😅

• Note on PGDATA changes for PostgreSQL 18+ #2582
• Update PostgreSQL documentation now that 18 is GA #2624

[phillipross]

DOH! Apologies... I created the issue against the wrong repo 😅 And thanks to all of you for documenting it!