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

Warning

1-column layout limitations

when in 2 panel mode, only folders in the right (sub-folders & assets) panel show notes on selection in the inspector.
when selecting a folder in the left (folder only) panel the inspector doesn't update, because unity doesn't register a selection change.

left red panel, no folder notes support. right green panel, supports folder notes

swap to 2-column layout

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