Add system dependencies to README Installation section to prevent OpenCV setup errors

[chigwell]

User Story
As a developer setting up the project,
I want the README.md to clearly document all non-Python dependencies required for OpenCV (like ffmpeg)
so that installation errors related to missing system packages are prevented.

Background
The current README.md only lists Python dependencies via setup.py (e.g., opencv-python). OpenCV requires ffmpeg for video processing, which is not explicitly mentioned. This gap causes failures during VideoCapture initialization in video_describer.py for users without ffmpeg installed. New contributors and users report installation issues, especially on fresh environments.

Acceptance Criteria

• Update README.md under the "Installation" section to include:
  • A "System Dependencies" subsection listing ffmpeg as a prerequisite.
  • OS-specific installation commands (e.g., sudo apt-get install ffmpeg for Ubuntu, brew install ffmpeg for macOS).
  • A note explaining that missing ffmpeg may cause cv2.VideoCapture failures in video_describer.py.
• Validate that the updated README.md:
  • Uses clear headings and code formatting for commands.
  • Links to official ffmpeg installation guides for troubleshooting.
  • Matches the tone/style of existing documentation (e.g., concise bullet points).