Is it possible to add the note widget to Mac OS X Yosemite’s Notifications 'Today"’ tab? If so how?
You might consider taking notes in Sorter.
Until you pointed it out I did not even know that your could do such a thing with Sorter as I did not use Sorter much and so did not have it open, I do now and n future I will use it for note-taking. Thanks for suggesting it. 
You don’t need to have Sorter open to use the take note feature. If you assign a hotkey to Take Note in the Sorter preferences, then that shortcut will launch the Take Note pane even if Sorter is turned off (hidden). I almost never use the Sorter, but I use the Take Note feature all the time. This assumes that DEVONthink is running-the shortcut does nothing if DEVONthink is not running and the Sorter is hidden.
There are two differences between Take Note in Sorter and the global hotkey that Greg mentioned. In Sorter, notes taken with Take Note can only be saved into folders that are configured as Sorter bins. With Take Note invoked from the global hotkey, one can save a note in any group of any open database, or even create a new group in a database and save the note there. If you are taking notes as part of a focused research project limited to a few specific groups, the Sorter-based Take Note might be convenient.
Be aware that when Sorter is active, the shortcut that Greg mentions will invoke Take Note in Sorter.
(The second difference is frivolous: the HUD display in Take Note in Sorter is less ugly than the Take Note panel invoked by the global hotkey. Granted - a “less ugly HUD” is a distinction without a difference 
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Thank you Greg and Korm, it gets better! Although I do agree with korm’s HUD comment, (it is the particular shade of grey that is not so nice) the hot key solution is ideal for me. Cheers guys