[examples][cleanup] add nicer headlines and description in example readme...

[examples][cleanup] add nicer headlines and description in example readme introducing the three examples

examples/README.md

 Examples
 ===============
-The following resources are useful to get started with DuMux:
+The following resources are useful to get started with DuMuX:
 
-* [1ptracer](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/tree/master/examples/1ptracer),
-* [2pinfiltration](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/tree/master/examples/2pinfiltration),
-* [shallowwaterfriction](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/tree/master/examples/shallowwaterfriction).
+ * [Example 1: One phase flow with a tracer model:](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/tree/master/examples/1ptracer)
+
+    This example contains a contaminant transported by a base groundwater flow in a randomly distributed permeability field.
+
+ * [Example 2: Two-phase flow with infiltration and adaptive grid:](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/tree/master/examples/2pinfiltration)
+
+    In this example we model a soil contamination problem where DNAPL infiltrates a porous medium. The initial distribution of DNAPL is known and we can read it from a txt-file. The grid is adapitvely refined around the injection.
+
+ * [Example 3: Shallow water model:](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/tree/master/examples/shallowwaterfriction)
+
+    This example shows how the shallow water flow model can be applied to simulate steady subcritical flow including bottom friction.