add benchmarks for PostgreSQL and MySQL

[kholdrex]

This pull request includes an update to the README.md file to provide more comprehensive performance benchmarks for SimpleQuery. The changes highlight performance improvements when using SimpleQuery with larger datasets and across different database systems.

Performance benchmark updates:

• Updated performance benchmarks to include results for 1,000,000 records on both PostgreSQL and MySQL.
• Added detailed performance results for PostgreSQL, showing that the Struct-based approach remains the fastest, and the Read model approach is significantly faster than ActiveRecord.
• Included performance results for MySQL, demonstrating similar trends with the Struct-based approach outperforming ActiveRecord and the Read model approach still being faster than ActiveRecord.