Update background-mode.md (#165)

src/user-guide/repo-settings/background-mode.md

@@ -43,3 +43,7 @@ If credentials are required for accessing the repository, you will have to maint
 * To periodically run the background logic, you can define a job with transaction `SM36`.
 * Create a step for the program `ZABAPGIT` (or `ZABAPGIT_STANDALONE`, depending on which version of abapGit you are using). This will execute *all* defined background activities.
 * You can check the log of the job with transaction `SM37`
+
+### Custom background modes
+
+Implement interface `ZIF_ABAPGIT_BACKGROUND` in a global class, and it will automatically show up in the abapGit background settings, making it possible to configure and run for a repository.