Add Cesium GS to CONTRIBUTORS.md

Author: pjcozzi

CONTRIBUTORS.md

@@ -122,6 +122,8 @@ See [CONTRIBUTING.md](CONTRIBUTING.md) for details on how to contribute to Cesiu
    * [Daniel Cooper](https://github.com/moodragon46)
 * [GeoFS](https://www.geo-fs.com)
    * [Xavier Tassin](https://github.com/xtassin/)
+* [Cesium GS, Inc.](https://cesium.com/)
+   *
 
 ## [Individual CLA](Documentation/Contributors/CLAs/individual-cla-agi-v1.0.txt)
 * [Victor Berchet](https://github.com/vicb)