finish description of Prerequisits

Usecases/README.md

@@ -35,6 +35,8 @@ To understand how usecases are set up, let's have a look at the file
 [Two-Phase-Richards/two-patch/TP-R-two-patch-test-case/](./Two-Phase-Richards/two-patch/TP-R-two-patch-test-case/).
 Each script is broken into certain parts, which we explain in what follows.
 
+After the license notice and an importblock, the first block is
+
 ### PREREQUISITS
 ~~~python
 # PREREQUISITS  ###############################################################
@@ -55,3 +57,15 @@ use_case = "TPR-2-patch-realistic-testrun"
 # The name of this very file. Needed for creating log output.
 thisfile = "TP-R-2-patch-test.py"
 ~~~
+The PREREQUISITS block contains a routine that checks whether or not the
+directory  `output/`  in the same folder than the script exists and creates it
+in case it doesn't.
+The two variables that should be eddited by the programmer are `use_case`
+and `thisfile`.
+- `use_case`  contains the name of the name of your usecase. This name is
+used for `stdout`, but also for creating the output directory for the
+simulation data. Chose a descriptive name for the usecase you are simulating.!
+- `thisfile` should be the name of the script file. This variable is used to
+output the content of the script to stdout and into the logfile if the script
+`run_simulation` is used to start the simulation. The latter should be
+contained in all folders.