Jupyter Notebook supports Markdown syntax for organized documentation:

1. Select Markdown from the drop-down list in the toolbar.
2. Write your notes in the gray input box.
3. Click on Run or Shift + Enter:

After running the cell, the text will be styled in the display:

You can find a detailed Markdown cheat sheet by Adam Pritchard at https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet.