[readme] add text for exercise

exercises/exercise-mainfile/README.md

@@ -13,7 +13,7 @@ To summarize the problems differ in:
 * exercise1_1p_b: a one-phase compressible, stationary problem
 * exercise1_1p_c: a one-phase compressible, instationary problem
 
-The problem set-up for all three examples is always the same: It is a two dimensional problem and the domain is $`1 m x 1 m`$. It is a heterogeneous set-up with a lens in the middle of the domain which has a lower permeability ($`1\cdot 10^{-12} m^2`$ compared to  $`1\cdot 10^{-10} m^2`$ in the rest of the domain).
+The problem set-up for all three examples is always the same: It is a two dimensional problem and the domain is $`1 m`$ by $`1 m`$. It is a heterogeneous set-up with a lens in the middle of the domain which has a lower permeability ($`1\cdot 10^{-12} m^2`$ compared to  $`1\cdot 10^{-10} m^2`$ in the rest of the domain).
 
 <img src="https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-course/raw/master/exercises/extradoc/exercise1_1p_setup.png" width="1000">
 
@@ -47,6 +47,8 @@ The general structure of any main file in DuMux is:
 // define the type tag for this problem
 using TypeTag = TTAG(OnePCompressible);
 ```
+The TypeTag is created in the `1pproblem.hh`. There you can see that it inherits from the __OneP__ and additionally from the __CCTpfaModel__ which defines the discretization method, which is in this case the cell-centered tpfa method.
+
 * a gridmanager tries to create the grid either from a grid file or the input file
 
 ```c++
@@ -179,3 +181,6 @@ For the incompressible one phase problem it is possible to also have an analytic
 ```c++
 // TODO: dumux-course-task
 ```
+For the analytic solution of your immiscible problem you need analytic solutions for the derivatives of the jacobian. For that we have a special local residual, the `OneincompressibleLocalResidual` which provides that. You just need to include that in your `1pproblem.hh` and use that instead of the `immisciblelocalresidual.hh` which is used as a standard for all immiscible models.
+
+Additionally you need to set the differentiation method in the main file `exercise1_1p_a.cc` to analytic.