Improve-Coverage.md

+To obtain the detailed code coverage report, you can either
+- login to gitlab, select the latest CI pipeline, select the `coverage` job (run it, if not yet done), click on "Download", unpack and store the coverage report
+- locally, run 
+   ```
+   cmake .. -DCMAKE_BUILD_TYPE=Coverage -DUSE_Pythia8_C8=C8
+   make -j4 && ctest -j4 && make coverage
+   ```
+
+In both cases, the coverage is accessible in `coverage-report/index.html`.
+
+Recommendation:
+
+If you create a new code with a method/function call, anything like:
+```
+double newFunctionA();
+class newClass { public: int newMethodB(); };
+```
+You must provide a unit-test that calls and tests the new functionality:
+```
+TEST_CASE("NewTest") {
+  double resultA = newFunctionA();
+  CHECK(resultA == Approx(expected_resultA));
+
+  newClass obj;
+  int resultB = obj.newMethodB();
+  CHECK(resultB == Approx(expected_resultAB));
+}
+```
+Note, it is in particular important, that for any branch in your code (`if`, `switch`, etc.) you must provide suitable input parameters for the unit-test to go all the branch options. Make sure
+you always `CHECK` expected behavior!
+
+
+See more details on units test: [Guidelines for Unit Tests](Guidelines for Unit Tests).
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