explain helper functions

LDDsimulation/README.md

@@ -11,7 +11,8 @@ It defines the class `LDDsimulation` that contains methods to setup the
 simulation and run it. Each usecase file in `../Usecases` essentially
 sets up an instance of this class and defines
 all parameters needed by the `LDDsimulation.set_parameters(kwrgs**)` function
-which gets called 
+which gets called by the `main()` of each usecase (more precisely by
+  `helpers.run_simulation()`).
 
 ## `domainPatch.py`
 
@@ -24,3 +25,16 @@ which gets called
 ##  `functions.py`
 
 ## `helpers.py`
+
+This file contains some helper functions which are mostly used to keep the usecase files tidier.
+It should be clear from the function names what each function is used for.
+The following is noteworthy if you start developing:
+
+- `run_simulation(**kwrgs)`: This function gets called by the main of each usecase and sets up and runs an instance of `class LDDsimulation`.
+_Note that if you add features to the latter, say you add parameters or generally change the parameter structure, this function will most likely have
+to be changed accordingly._
+
+- `generate_exact_solution_expressions(**kwrgs)`: This function generates the
+initial condition, exact solution as well as corresponding source term dictionaries given a symbolic exact solution expression as input. This means that _the models of the code are hardcoded_ in this function.
+_If you want to add other models of the code and test with exact solutions,
+this function will have to be adjusted!_