Adding documentation link for object based motion blur

dist/preview release/what's new.md

@@ -4,7 +4,7 @@
 
 - Added support for [parallel shader compilation](https://www.khronos.org/registry/webgl/extensions/KHR_parallel_shader_compile/) ([Deltakosh](https://github.com/deltakosh))
 @NEED DEMO- Added FlyCamera for free navigation in 3D space, with a limited set of settings ([Phuein](https://github.com/phuein))
-@NEED DEMO- Added Object Based Motion Blur post-process ([julien-moreau](https://github.com/julien-moreau))
+- Added [Object Based Motion Blur](http://doc.babylonjs.com/how_to/using_motionblurpostprocess) post-process ([julien-moreau](https://github.com/julien-moreau))
 @NEED DEMO- WebXR ([TrevorDev](https://github.com/TrevorDev))
   - Add customAnimationFrameRequester to allow sessions to hook into engine's render loop ([TrevorDev](https://github.com/TrevorDev))
   - camera customDefaultRenderTarget to allow cameras to render to a custom render target (eg. xr framebuffer) instead of the canvas ([TrevorDev](https://github.com/TrevorDev))