docs: add note about ffmpeg dependency and RuntimeWarning

[Saurabhtbj1201]

Subject: docs: add note about ffmpeg dependency and RuntimeWarning

Description:

What does this PR do?

This PR updates the README.md to clarify the system requirements for Audio transcription.

Currently, markitdown triggers a RuntimeWarning: Couldn't find ffmpeg or avconv... when users run conversions on systems without ffmpeg installed (since pydub attempts to look for it). Adding a brief note about this dependency saves new users from confusion when they see this warning text in the terminal during general CLI usage.

Changes

• Updated the list of supported formats in the README.md under the "Audio" section to mention ffmpeg/avconv prerequisites.
• Mentioned the RuntimeWarning directly so it can be easily found via search.

Checklist

• I have read the CODE_OF_CONDUCT.md and LICENSE agreements.
• I have run the pre-commit hooks locally to ensure formatting and linting pass.
• Existing tests run successfully.
• This is a documentation-only change; no new Python dependencies or code logic were introduced.