Replace README.txt with README.md and update README.md file

Thank you for the observation. We agree that switching to README.md makes sense to take advantage of Markdown rendering. If you can help us with that, it would be greatly appreciated. Thanks in advance!

I've created a MR, please review: https://git.drupalcode.org/project/lazy_iframe/-/merge_requests/1

Thank you! The corresponding merge request has already been created. We appreciate your contribution.