Add some missing stuff to README.md

[KaddaOK]

• there should be a section shouting out the dependencies, especially linto-ai/whisper-timestamped
• add something about contributing, I suppose
• also how to ask questions or raise issues
• it's long, so a table of contents would be nice
• understanding what something does is much easier with a bunch of screenshots, so maybe add some more of it actually running and/or put them earlier
• speaking of which, fix the spacing between the existing introduction screenshots, which was fine and readable in a local markdown editor with a dark background but unreadable on github with its light background
• ultimately it needs a demo video