Add URL examples for each content type to README.md

### Discussed in https://github.com/DominikPieper/obsidian-ReadItLater/discussions/149

<div type='discussions-op-text'>

<sup>Originally posted by **MarcelBigger** May 21, 2024</sup>
What do you have to place on the clipboard, for instance in regard to a Tweet? Its URL? Always the URL?</div>