Update CODEOWNERS (eu-digital-green-certificates#18)

Author: daniel-eder

CODEOWNERS

@@ -5,4 +5,4 @@
 # For more details, read the following article on GitHub: https://help.github.com/articles/about-codeowners/.
 
 # These are the default owners for the whole content of this repository. The default owners are automatically added as reviewers when you open a pull request, unless different owners are specified in the file.
-* @ascheibal @MykhailoNester
+* @ascheibal @MykhailoNester @joerg-oppermann-tsi