Expand section on system tests in Readme

Now includes command 'build_tests'

README.md

@@ -255,8 +255,15 @@ A debugger needs special security privileges usually not provided by the Docker
 
 The debugger `gdb` is pre-installed in the Docker container.
 
-### DORiE is running, but I suspect that something is wrong
-You can execute system tests in order to ensure that DORiE is running correctly and producing the expected results:
+### Running System Tests
+DORiE includes system tests for comparing its results the ones of ODE solvers or
+former versions of itself. They ensure that DORiE is running correctly and
+producing the expected results. If you installed DORiE, you have to make sure
+that all test executables are built by calling
+
+    dunecontrol --only=dorie make build_tests
+
+You then execute the system tests with
 
     ARGS="--output-on-failure" dunecontrol --only=dorie make test