added clean code

documentation/coding-conventions.adoc

@@ -56,6 +56,14 @@ As a general goal, the code should be easy to read and understand. Besides, clea
 * To document implementation details, we use code comments (e.g. `// we have to flush explicitly to ensure version is up-to-date`). This is only needed for complex logic.
 * Avoid the pointless `{@inheritDoc}` as since Java 1.5 there is the `@Override` annotation for overridden methods and your JavaDoc is inherited automatically even without any JavaDoc comment at all.
 
+== Clean Code
+The book https://www.pearson.de/clean-code-a-handbook-of-agile-software-craftsmanship-9780132350884[Clean Code] gives a collection of helpful rules and guidelines for coding.
+A https://gist.github.com/wojteklu/73c6914cc446146b8b533c0988cf8d29[summary] can be found here.
+
+Most important explicit aspects are:
+
+* Do not push out-commented code (see also https://rules.sonarsource.com/java/RSPEC-125/[RSPEC-125] rule from SonarQube)
+
 == Catching and handling Exceptions
 When catching exceptions always ensure the following: