Update what's new.md

dist/preview release/what's new.md

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 - New [AnimationGroup class](http://doc.babylonjs.com/how_to/group) to control simultaneously multiple animations with different targets ([deltakosh](https://github.com/deltakosh))
 
 ## Updates
+- VRHelper now has onSelectedMeshUnselected observable that will notify observers when the current selected mesh gets unselected
+  ([carloslanderas](github.com/carloslanderas))
 - New watcher configuration for VSCode. Now the task only compiles changed files ([sebavan](https://github.com/sebavan))
 - Added new draw modes to engine (points, lines, linesloop, linestrip, trianglestrip, trianglefan) ([benaadams](https://github.com/benaadams))
 - Added GUI Textblock.lineSpacing setter and getter to configure vertical space between lines in pixels or percentage values when working with text wrapping ([carloslanderas](github.com/carloslanderas))