Improve README readability.

## Add Table of contents on README
README.md was long so I add **table of contents** which help user find what they want. 

## Add tips section 
Because users want to download videos and Tips help them to use it easily.

Thanks in advance. 
This is misleading; there's nothing wrong with `youtube-dl -x --audio-format mp3 https://www.youtube.com/watch?v=nfWlot6h_JM`. In any case, there's lots of fluff text and barely any content in this paragraph, so I'd rather leave it out.