Add detailed usage docs for writing queries and exporting results

[muk2]

Summary

• Added a "Writing and Running Queries" section to README.md explaining query execution, multi-statement editing, cursor-based execution, and semicolon splitting behavior
• Clarified that pgrsql uses standard PostgreSQL SQL syntax with no special formatting or spacing requirements — addresses confusion reported in Query editor misinterprets valid SQL syntax #57 and Query formatting rules unclear and inconsistent #60
• Added a dedicated "Exporting Results" section documenting all 5 export formats (CSV, JSON, SQL INSERT, TSV, clipboard) and the export workflow

What Changed

• README.md: Added two new documentation sections between the existing "Query History" and "Examples" sections

Test Plan

• Read through the new sections for clarity and accuracy
• Verify the documented keyboard shortcuts match actual behavior (F5, Ctrl+S, 1-5 quick export)
• Verify the documented export file naming convention (pgrsql_export_YYYYMMDD_HHMMSS.<ext>) matches the code

Closes #58
Related: #57, #60

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