• put FolderNoteEditor.cs editor script in editor folder
• select a folder in the Project tab, and write a note in the inspector tab.
  

Note

when in 2 panel mode, only folders in the asset panel will show notes on selection in the inspector.
when selection a folder in the folder only panel the inspector doesn't update, because unity doesn't register a selection change.

You can swap your Project tab to 1 panel view mode, by clicking on the vertical ...