Actually, it says so right in the 1st note "Getting started": "Click inside a new note and start typing. Press “shift+return” (⇧⏎) to create a new cell and use

Actually, it says so right in the 1st note "Getting started": "Click inside a new note and start typing. Press “shift+return” (⇧⏎) to create a new cell and use “backspace” to merge cells. That’s all you need to know to start using Quiver."