Add comprehensive usage documentation to README

[kacy]

Summary

• Adds detailed usage examples for client initialization, authentication, and database operations
• Documents all available filter methods with their PostgREST equivalents
• Provides complete code examples that users can copy and adapt

Changes

• Client initialization (environment variables and explicit configuration)
• Authentication examples (signup, signin, token refresh, JWT validation)
• Database CRUD operations with the fluent query builder
• Filter methods reference table
• Query modifiers documentation
• Complex query example combining multiple filters