add animationGroup documentation

dist/preview release/what's new.md

@@ -2,7 +2,7 @@
 
 ## Major updates
 - Introduced texture binding atlas. This optimization allows the engine to reuse texture bindings instead of rebinding textures when they are not on constant sampler indexes ([deltakosh](https://github.com/deltakosh))
-- New [AnimationGroup class](doc here) to control simultaneously multiple animations with different targets ([deltakosh](https://github.com/deltakosh))
+- New [AnimationGroup class](http://doc.babylonjs.com/how_to/group) to control simultaneously multiple animations with different targets ([deltakosh](https://github.com/deltakosh))
 
 ## Updates
 - New watcher configuration for VSCode. Now the task only compiles changed files ([sebavan](https://github.com/sebavan))