Update what's new.md

dist/preview release/what's new.md

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 # 2.6.0:
 
 ### Major updates
+ - New Unity 5 Editor Toolkit. Complete pipeline integration [Doc](TODO) - ([MackeyK24](https://github.com/MackeyK24))
  - New DebugLayer. [Doc](TODO) - ([temechon](https://github.com/temechon))
  - New `VideoTexture.CreateFromWebCam` to generate video texture using WebRTC. [Demo](https://www.babylonjs-playground.com#1R77YT#2) - (Sebastien Vandenberghe)(https://github.com/sebavanmicrosoft) / ([deltakosh](https://github.com/deltakosh))