Merge remote-tracking branch 'refs/remotes/origin/master'

Add an explanatory comment on why I placed the file saving and loading code in a class extension file.

Create README

Remove the NSToolbarItem subclass file, ToolbarButton.swift, from the project. I went with image toolbar items to add and remove note to avoid subclassing.

Replace the toolbar buttons with image toolbar items so I can use the same code to validate the Delete Note toolbar item that I used to validate the Delete Note menu item. I can also get rid of the NSToolbarItem subclass.

Add a comment telling people where to validate additional menu items.

Clean up some formatting in the switch statement.

Override the validate function to disable the Delete Note toolbar button when there are no notes.

Set the class of the Delete Note toolbar button to my ToolbarButton subclass.

Add a ToolbarButton subclass of NSToolbarItem so I can disable the Delete Note button if the notes array is empty.

Validate the user interface item so the Delete Note menu item is enabled only if there are notes to delete.

Add a class extension file to SplitViewController to validate the Delete Note menu item so it's enabled only if there are notes to delete.

Remove the code for the function registerForNotifications. I set the text view's delegate to the text view controller in the storyboard. By setting the delegate, the text view controller can handle the textDidChange notification without having to add an observer to the notification center.

Make the text view controller the delegate for the text view. By doing this the text view controller does not have to manually observe the textDidChange notification. I can remove the code registering for the notification.

Add a comment to explain the ?? clause in the read statement.