added a PiTest section to HOWTO.md

README.md

@@ -98,3 +98,44 @@ To create and push a new tag use:
 After you've moved an existing the tag to another commit, you can use:
 
     git push origin HOWTO-... --force
+
+## Special Build Tasks
+
+Besides common build tasks like `build`, `test` or `bootRun` this projects has some not so common tasks which are explained in this section.
+
+### Mutation Testing PiTest
+
+    ./gradlew pitest
+
+Runs (almost) all JUnit tests under mutation testing.
+Mutation testing is a means to determine the quality of the tests.
+
+#### Some Background Information on Mutation Testing
+
+PiTest does it with these steps:
+
+-   initially PiTest checks which production code is executed by which tests
+-   if the tests don't pass, it stops
+-   otherwise the production code is 'mutated' and PiTest checks whether this makes a test fail ('mutant killed')
+-   Finally it checks thresholds for coverage and mutant killing.
+
+More information about can be found here:
+
+-   PiTest: http://pitest.org/
+-   gradle-plugin: https://gradle-pitest-plugin.solidsoft.info/
+
+#### How to Configure PiTest
+
+These thresholds can be configured in `build.gradle`,
+but we should generally not lower these.
+
+There is also a list of excluded files, all generated by JHipster or MapStruct, not containing any changes by us.
+
+As you might figure, mutation testing is CPU-hungry.
+To limit load in our Jenkins build server, it only uses 2 CPU threads, thus it needs over an hour.
+
+If you want to spend more CPU threads on your local system, you can change that via command line:
+
+    ./gradlew pitest -Doverride.pitest.threads=7
+
+I suggest to leave one CPU thread for other tasks or your might lag extremely.