add example for slicer

Polygon_mesh_processing/doc/Polygon_mesh_processing/examples.txt

@@ -10,5 +10,6 @@
 \example Polygon_mesh_processing/polygon_soup_example.cpp
 \example Polygon_mesh_processing/triangulate_polyline_example.cpp
 \example Polygon_mesh_processing/refine_fair_example.cpp
+\example Polygon_mesh_processing/mesh_slicer_example.cpp
 
 */

Polygon_mesh_processing/examples/Polygon_mesh_processing/mesh_slicer_example.cpp

@@ -0,0 +1,6 @@
+#include <CGAL/Polygon_mesh_slicer.h>
+
+int main()
+{
+  return 0;
+}