Merge pull request #282 from AnalyticalGraphicsInc/pjcozzi-patch-1

Add link to "Cesium's Participation in OGC Testbed 13" blog post

README.md

@@ -104,6 +104,7 @@ Also see the [3D Tiles Showcases video on YouTube](https://youtu.be/KoGc-XDWPDE)
    * 3D Tiles motivation and ecosystem update ([pdf](https://cesium.com/presentations/files/3D-Tiles-OGC-DC.pdf)) at the OGC Technical Committee Meeting (March 2016).
    * 3D Tiles intro ([pdf](https://cesium.com/presentations/files/SIGGRAPH2015/Cesium3DTiles.pdf)) at the Cesium BOF at SIGGRAPH 2015.
 * **Selected Articles**
+   * [Cesium's Participation in OGC Testbed 13](https://cesium.com/blog/2018/02/06/citygml-testbed-13/). February 2018
    * [Adaptive Subdivision of 3D Tiles](https://cesium.com/blog/2017/08/11/Adaptive-Subdivision-of-3D-Tiles/). August 2017.
    * [Aerometrex and 3D Tiles](https://cesium.com/blog/2017/07/26/aerometrex-melbourne/). July 2017.
    * [Duke Using 3D Tiles for Excavation in Vulci](https://cesium.com/blog/2017/05/22/duke-vulci-photogrammetry/). May 2017.