Add check for DOM extension

[MatzeKitt]

What feature are you requesting?

• [x] Check if class DOMDocument is available and bail with an error message if it is not.
• [x] Add requirement to readme

Why is this feature necessary?

The DOM extension is necessary for Embed Privacy to work.

Are you expecing side-effects?

No response

Code of Conduct

• [X] I agree to follow this project's Code of Conduct