Update doc/ci/git_submodules.md

doc/ci/git_submodules.md

@@ -64,3 +64,14 @@ If you use the [`CI_JOB_TOKEN`](jobs/ci_job_token.md) to clone a submodule in a
 pipeline job, the user executing the job must be assigned to a role that has
 [permission](../user/permissions.md#gitlab-cicd-permissions) to trigger a pipeline
 in the upstream submodule project.
+
+## Troubleshooting
+
+### Can't find the `.gitmodules` file
+
+The `.gitmodules` file might be hard to find because it is usually a hidden file.
+You can check documentation for your specific OS to learn how to find and display
+hidden files.
+
+If there is no `.gitmodules` file, it's possible the submodule settings are in a
+[gitconfig](https://www.atlassian.com/git/tutorials/setting-up-a-repository/git-config) file.