add wmts issue in cesium

cesium-integration.md

@@ -15,6 +15,14 @@ Ongoing work:
 * 1. POC: http://bl.ocks.org/tonio/raw/9759909/
 * To be developed further: http://bl.ocks.org/gberaudo/raw/51f050326d91d4e9c09a/1b55ad64a1f3de2dcba5673ecb52f976104a4592/#
 
+## Sync raster data sources
+Idea: If a user adds a WMS/TMS/WMTS layer to ol3.js map it should also be added to the cesium globe.
+
+Known limitations: for now, ol3js and cesium don't support the same layer type:
+ * ol3js: WMTS, WMS, OSM, Bing
+ * cesiumjs: WMS, TMS, OSM, Bing, ESRI
+
+WMTS issue, see: https://github.com/AnalyticalGraphicsInc/cesium/issues/1890
 
 ## Share vector data
 Idea: Bring vector data the user interact with in 2d into 3d. Allow same styling, labels and tooltip information.