Ask user to open issue discussing third-party

Author: OmarShehata

CONTRIBUTING.md

@@ -79,7 +79,7 @@ Our code is our lifeblood so maintaining CesiumJS's high code quality is importa
       * Update [CHANGES.md](CHANGES.md).
       * Include reference documentation with code examples.  Follow the [Documentation Guide](Documentation/Contributors/DocumentationGuide/README.md).
       * If the change is significant, add a new [Sandcastle](http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html) example or extend and existing one.
-   * If you added third-party libraries, including new version of existing libraries, update [LICENSE.md](LICENSE.md).  Mention it in [CHANGES.md](CHANGES.md). 
+   * If you added third-party libraries, including new version of existing libraries, update [LICENSE.md](LICENSE.md).  Mention it in [CHANGES.md](CHANGES.md). If you plan to add a third-party library, start a [GitHub issue](https://github.com/AnalyticalGraphicsInc/cesium/issues/new) discussing it first.
 
 ## Code of Conduct