Use the horizontal line button in the toolbar to add a visible horizontal line between paragraphs.
To add a line, place the cursor where you want the line to go, then click the horizontal line button.
Example
Lorem ipsum dolor sit amet, consectetur adipiscing elit. In libero massa, rhoncus vitae turpis eget, facilisis mattis arcu. Suspendisse at tellus a arcu pulvinar sollicitudin. Nunc pellentesque lectus urna, at hendrerit risus hendrerit non. Morbi tincidunt fringilla mi a feugiat. Morbi faucibus consectetur nibh et lacinia. Quisque suscipit convallis arcu, sit amet auctor velit venenatis eget.
Ut fermentum arcu eget aliquet efficitur. Integer in dui dignissim, pulvinar justo at, euismod massa. Quisque convallis lobortis ligula, sed posuere diam tincidunt eget. Sed imperdiet convallis lectus, eu suscipit nisi fringilla at. Fusce ultricies ultricies purus, at elementum nisl.