Adding reference to doc for babylon.core.js

dist/preview release/what's new.md

@@ -7,6 +7,7 @@
       - New .obj serializer ([BitOfGold](https://github.com/BitOfGold))
     - Sprites now can be [picked](http://www.babylonjs-playground.com/#1XMVZW#3) and can use [actions](http://www.babylonjs-playground.com/#9RUHH#3) ([deltakosh](https://github.com/deltakosh))
     - New `Mesh.CreatePolygon()` method ([jerome](https://github.com/jbousquie))
+    - Introducing [babylon.core.js](http://doc.babylonjs.com/generals/Framework_versions) ([deltakosh](https://github.com/deltakosh))
   - **Updates**
     - Added `Scene.constantlyUpdateMeshUnderPointer` to improve performance when moving mouse ([deltakosh](https://github.com/deltakosh))
     - Added `StandardMaterial.disableLighting` ([deltakosh](https://github.com/deltakosh))