start explaining usecases

Usecases/README.md

+# Usecases
+
+In this folder and its various subfolders several usecases of LDD simulations
+can be found.
+The structure is as follows.
+
+- [Richards-Richards](./Richards-Richards)
+- [Two-Phase-Richards](./Two-Phase-Richards)
+- [Two-Phase-Two-Phase](./Two-Phase-Two-Phase)