Update CHANGES.md.

Author: bagnell

CHANGES.md

@@ -14,6 +14,7 @@ Change Log
 * Fixed a bug that could cause incorrect normals to be computed for exaggerated terrain, especially for low-detail tiles. [#3904](https://github.com/AnalyticalGraphicsInc/cesium/pull/3904)
 * Fixed a bug that was causing errors to be thrown when picking and terrain was enabled. [#3779](https://github.com/AnalyticalGraphicsInc/cesium/issues/3779)
 * Fixed issue where labels were disappearing. [3730](https://github.com/AnalyticalGraphicsInc/cesium/issues/3730)
+* Added `CullingVolume.fromBoundingSphere`.
 
 ### 1.21 - 2016-05-02