explain requisits

Usecases/README.md

@@ -31,4 +31,27 @@ and update that copy with information in the obsolete scripts.
 In order to create a new usecase, either modify one of the existing ones, or
 copy one of the existing cases into a new file and start working on your own.
 To understand how usecases are set up, let's have a look at the file
-[TP-R-2-patch-test.py](./Two-Phase-Richards/two-patch/TP-R-two-patch-test-case).
+`TP-R-2-patch-test.py` in
+[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.
+
+### PREREQUISITS
+~~~python3
+# PREREQUISITS  ###############################################################
+# check if output directory "./output" exists. This will be used in
+# the generation of the output string.
+if not os.path.exists('./output'):
+    os.mkdir('./output')
+    print("Directory ", './output',  " created ")
+else:
+    print("Directory ", './output',  " already exists. Will use as output \
+    directory")
+
+date = datetime.datetime.now()
+datestr = date.strftime("%Y-%m-%d")
+
+# Name of the usecase that will be printed during simulation.
+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"
+~~~