Documentation: pg_restore from heroku

[dennyluan]

To locally restore from a 14.7 pg_dump backup, postgres complains about missing heroku_ext

psql poster_development -c "CREATE SCHEMA IF NOT EXISTS heroku_ext"
psql poster_development -c "ALTER database poster_development SET search_path TO heroku_ext,public"
pg_restore --verbose --clean --no-acl --no-owner -h localhost -d poster_development <backup>