Add CODEOWNERS file (#86)

* Add CODEOWNERS file for automatic review requests, especially by Dependabot

* Update CODEOWNERS

Author: rfc2822

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+# See https://docs.github.com/de/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+
+# For combination with "Require review from code owners" for main branch:
+*      @bitfireAT/app-dev