Add sphinx gallery icosahedron thumbnail to README.md (#398)

README.md

@@ -36,7 +36,14 @@ pip install scikit-gmsh
 Check out the example galleries organized by subject here:
 
 <p align="center">
-  <a href="https://scikit-gmsh.readthedocs.io/en/latest/examples/polygon_with_hole.html"><img src="https://scikit-gmsh.readthedocs.io/en/latest/_images/sphx_glr_polygon_with_hole_thumb.png" height="200px"/></a> <a href="https://scikit-gmsh.readthedocs.io/en/latest/examples/cylinder.html"><img src="https://scikit-gmsh.readthedocs.io/en/latest/_images/sphx_glr_cylinder_thumb.png" height="200px"/></a>
+  <a href="https://scikit-gmsh.readthedocs.io/en/latest/examples/icosahedron.html">
+    <img src="https://scikit-gmsh.readthedocs.io/en/latest/_images/sphx_glr_icosahedron_thumb.png" height="190px"/>
+  </a>
+  <a href="https://scikit-gmsh.readthedocs.io/en/latest/examples/polygon_with_hole.html">
+    <img src="https://scikit-gmsh.readthedocs.io/en/latest/_images/sphx_glr_polygon_with_hole_thumb.png" height="190px"/>
+  </a>
+  <a href="https://scikit-gmsh.readthedocs.io/en/latest/examples/cylinder.html">
+    <img src="https://scikit-gmsh.readthedocs.io/en/latest/_images/sphx_glr_cylinder_thumb.png" height="190px"/>
 </p>
 
 ## Other Resources