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. 
The README serves as a basis for other files, namely the manpage. This PR includes the TOC unmodified in the manpage, which is neither useful (manpages are auto-indexed) nor correct (the manpage has some differences in the chapters).