Update README to reflect current PNG output and CLI flags

[arividar]

Summary

The README is outdated compared to the current implementation.

Details

• Code currently writes PNG files named YYYY-MM-DDTHH.MM.SS.png inside a date folder YYYY-MM-DD (see ScreenshotWriter::current_time_image_filename and date_folder_path).
• README mentions JPG output and positional arguments like scrabber C:\MyStuff\Screenshots and --Filename, which the code does not implement.
• CLI is defined with flags via clap (--path, --interval, --count, --forever, --skip-duplicates).
• Add documentation for --skip-duplicates and clarify default values.
• Ensure examples match the actual behavior and file layout.

Tasks

• Update README.md to:
  • Use PNG everywhere instead of JPG.
  • Show the date-based folder structure YYYY-MM-DD/ and filename format YYYY-MM-DDTHH.MM.SS.png.
  • Replace positional arg examples with the real flags (--path, etc.).
  • Document --skip-duplicates, --forever, and defaults.
  • Align example invocations with cargo run -- --help output.

Acceptance criteria

• README.md examples, flags, and output format match the current program behavior.
• Running cargo run -- --help aligns with README description.