Added a note about the total deprecation of YUI

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

Maybe we could also close the related issue #1841?
Yes, but Node.js is the one used in their [trademark policy](http://nodejs.org/images/trademark-policy.pdf).