[exercise-fluidsys] add density function plot to readme

exercises/exercise-fluidsystem/README.md

 # Exercise Fluidsystem
 
-The aim of this exercise is to get familiar with the _DuMuX_ way of implementing new components (fluids) and fluid systems (mixtures). In the scope of this exercise, a new fictitious component is implemented 
+The aim of this exercise is to get familiar with the _DuMuX_ way of implementing new components (fluids) and fluid systems (mixtures). In the scope of this exercise, a new fictitious component is implemented
 (exercise-fluidsystem a) as well as its mixture with water (exercise-fluidsystem b).
 
 ## Problem set-up
@@ -167,11 +167,11 @@ where $`p`$ is the pressure and $`\rho_{min} = 1440 `$, $`\rho_{max} = 1480 `$ a
 You can plot the density of the phase consisting of your compressible component by setting `PlotDensity` in `exercise-fluidsystem_a.input` to `true` and starting the simulation again.
 Compare the gnuplot output to the following plot of the density function from above:
 
-TODO
+![](../extradoc/exercise-fluidsystem_a_densityfunction.png)
 
 ### 3. Implement a new fluid system
 
-The problem file for this part of the exercise is `2p2cproblem.hh`. We now want to implement a new fluid system consisting of two liquid phases, which are water and the previously implemented compressible component. We will consider compositional effects, which is why we now have to derive our _TypeTag_ (`ExerciseFluidsystemTwoPTwoCTypeTag`) from a _TypeTag_ (`TwoPTwoC`) that holds the miscible two-phase 
+The problem file for this part of the exercise is `2p2cproblem.hh`. We now want to implement a new fluid system consisting of two liquid phases, which are water and the previously implemented compressible component. We will consider compositional effects, which is why we now have to derive our _TypeTag_ (`ExerciseFluidsystemTwoPTwoCTypeTag`) from a _TypeTag_ (`TwoPTwoC`) that holds the miscible two-phase
 two-component model properties:
 
 ```c++