[readme] Add model exercise

exercises/README.md

@@ -44,6 +44,16 @@ This exercise guides you through the following tasks:
 
 In this exercise, you learn how to adjust the properties in order to use a customized local residual.
 
+
+### [:open_file_folder: Model](./exercise-model/README.md)
+
+This exercise covers the implementation of new models DuMu<sup>x</sup>. You learn
+
+* how to design a nonlinear diffusion equation model from scratch (almost)
+* how to implement a local residual using the a minimal model design
+* use DuMu<sup>x</sup> to denoise an image (!)
+
+
 ### [:open_file_folder: Fluid systems](./exercise-fluidsystem/README.md)
 
 This exercise covers the handling of phases and components in DuMu<sup>x</sup>. You learn
@@ -52,6 +62,7 @@ This exercise covers the handling of phases and components in DuMu<sup>x</sup>.
 * how to implement a new fluidsystem,
 * and how to change the wettability of the porous medium.
 
+
 ### [:open_file_folder: Dune module](./exercise-dunemodule/README.md)
 
 You learn how to